Not just the code as it is now — the code plus a complete, queryable record of every change anyone ever made to get it there.
Before version control, “the code” was just whatever files happened to be on disk, with older versions surviving only if someone manually copied them somewhere first. A repository formalises this: it’s a directory of code tracked by a system that records every change as a discrete, attributable, reversible step, so the current state and the full history of how it got there are both always available. The idea is old enough to predate GitHub by four decades — an early 1975 system for controlling source code changes established the core mechanism that every modern repository, GitHub included, still builds on.
This is exactly the object this essay defines for its reader mid-sentence — “a repo or repository is a directory file of code stored typically on GitHub” — because everything else in the essay, cloning, forking, contributing to open-source projects, depends on the reader already having this concept.
Source: Rochkind, M.J. (1975), “The source code control system,” IEEE Transactions on Software Engineering, SE-1(4), 364–370.