Codebase
The complete body of source code behind a product, shared and changed by the whole team.
What it tells you about a developer
Developers effective in large existing codebases navigate unfamiliar systems and change them safely, which is the core skill of senior work at established companies. A history of substantial contributions to big established repositories evidences this directly.
A codebase is the accumulated work of everyone who ever touched the product, often millions of lines built over years. Most professional work is changing an existing codebase, which is why reading code takes up more of the job than writing it.