What is Repository
A project folder on GitHub holding the code plus the full history of every change ever made to it.
In plain terms
Everything on GitHub lives in repositories. A repository records who changed what and when, so individual credit inside a shared project stays traceable for years. Repositories are public, meaning anyone can read them, or private, meaning the owner and their invitees can.
What it tells you about a developer
Repositories a developer created themselves show what they build when nobody assigns them work. Original repositories with real documentation say more than forked copies.
Related
Read the lesson on Repository More in How software gets built