The "Source" is the project's files in the game engine editor that you have used to create the game you'll submit. For example, if you make the game in the Unity engine, you got to put the whole project folder which is required to make a "build" of your game (the client + files you submit) either with the game file or (which I highly suggest) as a separate link (such a Dropbox or Google Drive URL so that the judge can download the project and verify the content) given within your submission.
This is why the project must not contain ANY paid nor Free-To-Use under CC4.0 assets since the process of sharing a project's folder with paid assets is an considered as an unauthorized redistribution. (Any files you use has to be either made by you or available on the public domain, meaning no ownership by anyone in particular.)
The source files must work without any paid assets/plugins, but you can build stuff with paid tools as long as you fully remove those tools before submitting said the source files. To put it simple, the judges should be able to open your source file in your chosen engine without any error and should be able to build the exact same game as you submitted without anything missing. So you can't just use paid plugin/assets to make a build, remove them and submit your game with an incomplete source.