The Legacy is the collective term for the sum of all knowledge that the Silo retained of humanities history before the Silo was constructed. In its simplest form, the Legacy was a collection of books, relics, and artifacts housed within the Silo behind a locked vault; only the head of IT and their shadow could be aware of the Legacy at any given time.
Also contained within the Legacy was a computer program with access to further knowledge and books that were not housed physically within the Vault in the Silo. It is unknown, but presumed, that each Silo had their own 'Legacy', but it is not confirmed if their contents were the same. Solo appeared to be put in charge of guarding Silo-17's Legacy.