In context, the verse refers to counting the Omer. The full verse reads:
וּסְפַרְתֶּ֤ם לָכֶם֙ מִמָּחֳרַ֣ת הַשַּׁבָּ֔ת מִיּוֹם֙ הֲבִ֣יאֲכֶ֔ם אֶת־עֹ֖מֶר הַתְּנוּפָ֑ה שֶׁ֥בַע שַׁבָּת֖וֹת תְּמִימֹ֥ת תִּהְיֶֽינָה׃
You will count for yourselves from after the Sabbath, from the day on which the Omer of waving is brought; seven complete Sabbaths shall there be.
Rashi to the verse, quoting Menachos 66a, notes:
תמימת תהיינה. מְלַמֵּד שֶׁמַּתְחִיל וּמוֹנֶה מִבָּעֶרֶב, שֶׁאִם לֹא כֵּן אֵינָן תְּמִימוֹת
“Complete shall they be” - This teaches that one must begin counting in the evening, because if this is not the case, they will not be complete.
Given this teaching, it would appear that שבת as used in the verse refers to “weeks” rather than literally “Sabbaths.”
In summary, “complete” teaches that one must count at night; if he counts during the day, it can’t be considered complete, as he missed the previous night.