I'm done with my project but I was never able to know what any game jam means by submitting a source, does it mean source code, the sources any assets you used, the coding software?

Source means the files for the game. The user needs to be able to download your source and run it from the engine.

So... the entire folders I used for assets in Unity, for example?

If you used SourceTree for your version control I am now an "expert" at migrating it to GitHub after much trial and error last night.
If you need step by step instructions let me know.
Then you can just provide the link.