The source code and any assets that you have the rights or preference to share. Or rather, everything needed for someone else to build/compile/load the game. If there are assets you're not sharing for whatever reason, that's fine - I think it's helpful to make a note of it in the repository README.md file, and optionally provide alternative/placeholder assets :)
Example Unity project from 2017's Game Off called Neon Skylines might be a helpful to illustrate.