The marginal notes you are referring to are called the masorah, in particular the masorah parva (small or inner masorah). These mostly mark rare words or ketiv qerei pairs. The other two types are the masorah magna (big or outer) on the top and bottom of the pages, mainly with links to these rare occurrences. The last one is the masorah finalis (final) with the count of letters, words and verses. The main purpose of them were to avoid scribal errors and to guarantee the proper reading of these texts as handed down by the tradition.
I will do a line-by-line example with the top of the left column, which starts with Psalm 12:6:
יֹאמַ֣ר יְהוָ֑ה אָשִׁ֥ית בְּ֝יֵ֗שַׁע יָפִ֥יחַֽ־לֽוֹ׃
Nothing is marked, Mechon Mamre lacks a maqaf between the last two words.
אִ֥מֲר֣וֹת יְהוָה֮ אֲמָר֪וֹת טְהֹ֫ר֥וֹת כֶּ֣סֶף צָ֭רוּף
Mechon Mamre changes the chataf patach under the מ. On the right you can see this: 'ל'ל'ל. This is as abbreviation for ליתא, which means no other, meaning that it is a hapax legomenon, a word that only occurs here in the תנ"ך. Three words are marked with a small circle above: צָרוּף, אֲמָרוֹת, אִמְרוֹת. You can check them in Strong's Concordance, they are really hapax.
בַּֽעֲלִ֣יל לָאָ֑רֶץ מְ֝זֻקָּ֗ק שִׁבְעָתָֽיִם׃
בַּעֲלִיל is a hapax, שִׁבְעָתָיִם is marked with 'ב' מל, which means that 2 are in this final form with kamatz (out of its 6 occurrences marked with 'ו on the left), and both are written in the long male form with י.
אַתָּֽה־יְהוָ֥ה תִּשְׁמְרֵ֑ם תִּצְּרֶ֓נּוּ מִן־הַדּ֖וֹר
Hapax marked on the left: תִּצְּרֶנּוּ (note the disjunctive shalshelet gedolah),* while on the right you can see ג' ר"פ. Here you have to go down to the Masorah Magna, where you can see the three occurences at the beginning of a verse (Psalms 12:8, 40:12, Eikhah 5:19). I added the rest of the verses in parentheses:
אתה יהוה ג' ראש' פסוק' אתה יהוה תשמרם (תצרנו מן הדור זו לעולם) (אתה יהוה) לא תכלא רחמיך ממני (חסדך ואמתך תמיד יצרוני) אתה יהוה לעולם תשב (כסאך לדר ודור)
ז֣וּ לְעוֹלָֽם׃ סָבִ֗יב רְשָׁעִ֥ים יִתְהַלָּכ֑וּן כְּרֻ֥ם זֻ֝לּ֗וּת לִבְנֵ֥י אָדָֽם׃
Again three hapax marked with a circle: זֻלּוּת, כְּרֻם, יִתְהַלָּכוּן.
There are many other abbreviations as well, which you can find in this really good article from the Encyclopaedia Judaica.
* Thank you b a for pointing a previous mistake!