Repository map
A repository map is a compressed outline of a codebase, listing its files and the symbols they define, so a model can locate things without reading everything.
The purpose is orientation rather than content. A map answers the question of where something probably lives, which is what an agent otherwise spends several tool calls discovering.
Because it is an outline, it is small relative to what it describes: signatures rather than bodies, names rather than implementations. That ratio is the whole value.
A map does not replace reading the file that matters. It replaces reading the four that did not.