This is the oldest complete manuscript of the Hebrew Bible in existence. It was copied out by a scribe named Samuel ben Jacob, working in Old Cairo, who finished his work and dated it — by his own note at the end of the manuscript — to the year 1008 CE.
Samuel ben Jacob wasn't inventing a text; he was preserving one. He worked from the tradition of the Ben Asher family, the leading authorities on the Masorah — the elaborate system of vowel points, cantillation marks, and marginal notes that Jewish scholars had spent centuries refining to fix the Hebrew Bible's pronunciation and preserve it against corruption. An older, arguably more authoritative Ben Asher manuscript does survive — the Aleppo Codex — but much of its Torah section was lost in 1947. The Leningrad Codex is treasured precisely because it is complete, and old enough to sit right at the source of that tradition.
It takes its name from the city where it later came to rest — Leningrad, as St. Petersburg was called when the manuscript entered Western scholarship, a name that stuck even after the city's own name reverted in 1991. Today it underlies Biblia Hebraica Stuttgartensia, the standard scholarly edition of the Hebrew Bible — which means that almost every modern Old Testament translation, in almost every language, traces back to this one 11th-century Cairo scribe's careful copying, whether its readers ever know his name or not.