The Masorah: the counting of letters by which the text was kept מסורה

Where the word comes from. Masorah is from "to hand on." It names both the work and its fruit: the body of notes in the margins of scroll and codex by which the text is held unchanged.

Where it began. "Therefore the first sages were called soferim—counters—because they would count all the letters of the Torah" (Kiddushin 30a). The same passage names the middle letter of the Torah, the middle words and the middle verse. This is not curiosity: if it is known how many letters there should be and which stands in the middle, any omission is found by counting.

What the Masoretes did. They noted how often a word occurs and in what form; where one thing is written and another read (ketiv and keri); where a letter is larger or smaller than the rest; which verses begin alike. They wrote this in the margins in small script—the small Masorah at the sides and the great Masorah above and below.

Vowels and signs. Their work as well: before the Masoretes the text was written in consonants alone. The vowel points and the cantillation marks are a way of fixing a reading that until then was held only by mouth.

Tiberias. The chief school was that of Tiberias, the family of Ben Asher. Aaron ben Asher corrected the codex later called Keter Aram Tzova, the Aleppo Codex—by which the Rambam checked the laws of writing a scroll.

Why this stands in a historical timeline. Because the whole chronology of this reference work is counted from the text. If the text drifted, the count of years from Creation would drift with it.