So I am making a game in unity, and I want to understand what you are referring to as “source code”. Is this the code that I make for my game, or is it something else? Do I have to put the assets I made with the github post? Please reply to this, and I am looking forward to participate. Thanks.

Honestly i don't know myself, but i think the source code is the project itself. And yes you do need to upload it to the github repository.

Source code is: "a text listing of commands to be compiled or assembled into an executable computer program." - Google

This means the code for your game that gets compiled. 

To answer your second question, yes. You must post all assets. Like in my game: https://github.com/TheCrafters001/LOFBBBD

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.