The gemara in Chagigah 12a describes the primordial light of creation and says that with it, Adam could see from one end of the world to the other:
אמר רבי אלעזר: אור שברא הקדוש ברוך הוא ביום ראשון, אדם צופה בו מסוף העולם ועד סופו
Rabbi Elazar said: [With] the light that the Holy One, Blessed be He, created on the first day, Adam could see from one end of the world to the other
What does this mean? Is it pure metaphor, and if so what is the analog?
There are a few answers in addition to the יפה תואר quoted by @Y DJ:
1) The Shem Mishmuel (Parashat Tezaveh שנת תרע"ג) says that the light that was created on the first day, which allowed Adam Harishon to see from one end of the world to the other, does not mean that it was a very great light that enabled one to see to the furthest distance in a physical sense. After all, if the earth were flat, this would make sense, but the earth is spherical, and therefore, the earth's parts and its elevations would obstruct his view. Rather we must understand it like Tosafot Shabbat 22b ד״ה וכי לאורה quoting Braita of Malechet Hamishkan where it is mentioned that one could look at a pot and know what was inside, or at a barrel and know what was within. This vision is not a physical one, as physical objects obstruct other physical objects, but rather it is a spiritual vision, like seeing with Ruach Hakodesh where one sees and perceives as if the object were right in front of them, and physical matter does not obstruct spiritual perception. The source of this light lies in the deep attachment in an intense bond with Hashem. This is power given to creations, enables them to cling to such an intense bond. (similar to the Bal Shem Tov that @Rabbi Kaii quoted)
2) The Sefat Emet (Chanuka 5661 and Vayera 566) says a simialr idea to the Shem Mishmuel. Rashi on Shemot 3:14 says that the term Olam (world) is related to he’elim (to hide), indicating that the natural world conceals Hashem's presence to allow for free will. Ordinary light reveals only the physical aspects of objects, while the ohr haganuz not only showed physical properties but also the spiritual essence and Hashem's presence. This light allowed Adam Harishon to see from one end of the world to the other, revealing Hashem's guiding hand, involvment and care for the world beyond the natural world's concealment.
3) The Degel Machaneh Ephraim (Parshat Beresheit ד"ה וירא) says that the pasuk says, "וירא אלקים את האור כי טוב," the Midrash states that "good" means it was good to be hidden, and He hid it for the righteous. The righteous use this light in every generation. He heard from his grandfather that this light was hidden in the Torah (Zohar Chadash, Introduction, 1:264). Therefore, the righteous in every generation use this light, meaning through the Torah, which contains this light that allows them to see from one end of the world to the other. He says that he has personally witnessed several stories which where tzadikim truly saw from one end of the world to the other, and all this was through that light hidden in the Torah (he seems to understand the word אדם not just reffering to אדם הראשון, but tzadikim as well). The Bal Haturim (Beresheit 1:4) says this idea is is hinted at in the words, "את האור" which has the same numerical value as "בתורה," in the Torah.
Keter Shem Tov 2:53, paragraph 7 is a piece from the Baal Shem Tov. Paragraph 9 specifically (summary at the end):
ענין השבת הוא שמתגלה השורש והענפים חושקים אליו פי' שיום השבת הוא יום קדוש שמאיר ומתגלה בהירות מקדוש שהוא הש"י קדוש הקדושים ומאיר על הברואים. פי' כי ענין רוחניות של הכל הוא מה שנאצל במחשבה הקדומה וזהו חיות של הכל ואח"כ כשנברא הכל בפועל ע"י השתלשלות אעפ"כ אותו הרוחניות נשאר למעלה נעלם בשרשו והחיות שבברואים היה חיות קטן מאד שנתצמצם לשיוכל להתלבש בגופניות ואחר שנגמר הכל ביום ו' אלו היה העולם נשאר בזה הבריאה לא היה יכול להתקיים לזאת אחר גמר כל מעשה בראשית הבהיק הש"י בהירות מבריאה הנעלמה דהיינו ממה שהיו הברואים במחשבתו הוי' רוחניות מאד והבהיק זיו הדרו מסוף העולם עד סופו בכל מעשה בראשית והעיקר בהאדם הוא מבחר הברואים הש"י הבהיק אליו בהירות משרשו הנעלם שבמחשבת הש"י ב"ה וזהו ענין מה היה העולם חסר מנוחה ר"ל הש"י נק' מנוחה כו' בא שבת בא מנוחה. היינו בהירות הוייתם של הברואים הנעלמת שהוא מעצמותו ית' ואז נתמלאו חשק ורצון אליו כמו התינוק שהולך אחר מעשה נערות ושוכח באביו ואח"כ כשרואה את אביו מחמת חשקו אליו משליך הכל ומתדבק בו ורץ אליו מחמת שהוא נתח מנתחיו כן כביכול כשהש"י מבהיק זיו הדרו אל הברואים אז מגמת פניהם אליו בתשוקה גדולה וזה רצונו שמקוה מהם וזה סיבת קיומם וזה ענין השבת שהוא השבה אל השורש ר"ל השורש מאיר על הענפים והענפים חושקים ומתענגים בו ונכספים אליו והוא אחדות הש"י.
וז"ש בזהר רזא דשבת איהו שבת דאתאחדת ברזא דאחד דהא כורסייא יקירא ברזא דאחד פי' כי הברואים נקראים בכללות כסא להש"י כמ"ש השמים כסאי והברואים הם רבים מצד חצוניותם אבל מצד פנימיותם הם אחד ואימתי הם אחד כשהם דביקים וחשקם למחשבה אחת לדביקות הש"י נמצא כל מגמת חפצים רצון אחד וחשק אחד ולהדבק כאחד והדבקות בזה היא המאחדת ומקשרת הש"י עם הברואים לשיתקיימו ע"י הש"י שהוא אחד וזה ההתקשרות נק' ברית לזה נק' השב ברית עולם שאז הוא זווגא דקב"ה ושכינתי' שר"ל קב"ה נק' אלהות הנעלם מהברואים ושכינתי' הוא אלהות השוכן בתחתונים והתקשרות שניהם הוא ביום השבת. וכבר ידעת שתמיד כמו שהיה פעם א' מתעורר באותו הזמן תמיד ובפרט בעני יום השבת מוכרח להיות כן כי בטובו מחדש בכל יום תמיד מעשה בראשית וכל יום נברא יום א' ונברא ברואים בכל יום מדצח"מ עד בא יום השבת ומתעורר המקום ליתן חיות להם כמו שהיה בעת הבריאה
The concept of Shabbat is that the root is revealed, and the branches yearn for it. Shabbat is a holy day when the illumination from holiness is revealed, which is Hashem, the Holy of Holies, who shines upon the creatures. Meaning, the concept of the spirituality of everything is connected to the Primordial Thought, which is the life force of everything. Afterwards, when everything was created in reality through the spiritual cosmos (השתלשלות), that same spirituality remained above, concealed in its root. The life force in the creatures is very small, having been contracted to be able to be enclothed in physical bodies. After everything was completed on the sixth day, if the world had remained in this state of creation, it would not have been able to endure. [Therefore], after the completion of all the acts of creation, Hashem shone a bright light from this part of the creation that was concealed, which is the state in which the creatures existed in His primordial thought, a very spiritual state. This ray shone from one end of the world to the other, over all creation. The main recipient of this light was man, the most select of creatures. Hashem shone a bright light upon him from the hidden root within the Primordial Thought of Hashem. This is the meaning of "What was the world lacking? Rest," (Bereshit Rabbah 10:9) meaning Hashem is called "Rest." When Shabbat arrived, Rest arrived. This means that the hidden reality of the creatures, which is from His Essence, blessed be He, shone. Then they were filled with yearning and will towards Him, just as a child who follows childish activities, and forgets his father, but when he sees his father, he throws everything away and clings to him, running towards him because he is a part of his father, and because he yearns for him. Similarly, when Hashem shines the ray of His splendor on the creatures, their faces are turned towards Him with great need. This is what He hopes for and desires from them, and this is the reason for their existence. This is the concept of Shabbat, which is the arrival at the root, meaning the root shines oon the branches, and the branches long for and delight in Him, yearning for Him. This is the unity of Hashem, blessed be He.
As it is said in the Zohar (II 135a), "The mystery of Shabbat is that it is united with the mystery of oneness, for the precious throne is united in the mystery of oneness." Meaning, the creatures are collectively called the throne of Hashem, blessed be He, as it is written, "The heavens are My throne" (Yishayahu 66:1). The creatures are many in their external aspect, but in their internal aspect, they are one. When are they one? When they are attached and their longing is for one purpose – to cling to Hashem, blessed be He. Thus, all their desires and pursuits are for one will and one desire – to cling to Him as one. This attachment is what unifies and connects Hashem, blessed be He, with the creatures, so that they may endure through Hashem, blessed be He, who is one. This connection is called a covenant, which is why it is called an eternal covenant, for then there is a union between Hashem, and His Shechinah. Hashem, blessed be He, is the Divinity that is hidden from the creatures, and His Shechinah is the Divinity that dwells among the lower beings. Their connection happens on Shabbat. It is already known that what happened once is always aroused at that time, and especially on Shabbat, it must be so because in His goodness, He renews the work of creation constantly, and each day is a new creation, with new creatures of the inanimate, vegetative, living, and speaking realms, until Shabbat arrives, and HaMakom is awakened to give life to them, as it was at the time of their initial creation.
To extract from this stunning piece what is relevant, in my own words and understanding:
Our true reality - the true reality of all creations, and especially mankind - is ultimately lofty: clinging to Hashem's very Primordial Thought; the inner of the inner of His Divinity. As such, we are all truly one with Him in our internal essence, and indeed the whole world, from end to end, is one as a result.
However, this reality is hidden from us when we are in a body, in the same way that a child gets lost in his childish games and forgets about his father.
The "light" that shone is a metaphor for the revelation to the lower beings of the lofty truth of our existence, and the experiece of that revelation is similar to the experience of a child who, in the middle of his game, suddenly sees his father, and immediately yearns for him, and drops everything to go run after him.
When this light shines, we experience great desire and need for Hashem, fulfilling His own desire and need; the purpose of creation. This experience illuminates for us the true nature of creation - that it is all one with Hashem, and thus we "see from one end of the world to another", in a single glance.
The commentary יפה תואר / Yafe Toar explains:
הדרשנים אומרים שאור זה רמז להשגת השכל, שהיה משכיל ראשונה כל חלקי המציאות ואחר כך קצרה לסיבת נטיית המפורסמות, ובפרט לדור המבול.
The darshanim say that it is a hint to the original ability of his mind to perceive all parts of reality (i.e he had no need to break reality in parts to perceive its complete essence). Later it was diminished due to the tendency towards popular opinions, particularly in the generation of the flood.
For example, during the generation of the flood, Bereshis 6:2, we learn that the sons of princes and rulers (Genesis Rabbah 26:15) imposed their morality on the rest of humanity.
Also, there is a change in the language of the midrash. In Bereshis Rabba 11:2
אָמַר רַבִּי יְהוּדָה בַּר רַבִּי סִימוֹן, אוֹר שֶׁבָּרָא הַקָּדוֹשׁ בָּרוּךְ הוּא בְּיוֹם רִאשׁוֹן, אָדָם צוֹפֶה וּמַבִּיט בּוֹ מִסּוֹף הָעוֹלָם וְעַד סוֹפוֹ,
and in 12:6
אָמַר רַבִּי יְהוּדָה בַּר סִימוֹן, אוֹתָהּ הָאוֹרָה שֶׁנִּבְרָא בָּהּ הָעוֹלָם, אָדָם הָרִאשׁוֹן עָמַד וְהִבִּיט בָּהּ מִסּוֹף הָעוֹלָם וְעַד סוֹפוֹ
It notes, decreasing of the magnitude of light, from male אוֹר = "daylight" to female אוֹרָה - "light illuminating darkness"; as well ability of Adam to foresee צוֹפֶה וּמַבִּיט from before the sin and only gaze הִבִּיט after the sin.
