I assume we are talking about LibreCAD 2, so the link to the LibreCAD 3 thread might be irritating.
GitHub is just the platform for git. And Git is the tool for source code management. It tracks changes and allows to split and merge the code base for development.
If you don't have skills in Git, you can simply download a ZIP archive of the whole code (green Code button).
To get used to the LibreCAD code, you need C++ skills.
I recommend Qt Creator as IDE, because LibreCAD 2 is built with Qt framework, and Qt Creator has best support for it.
Armin
investing less than half an hour into Search function can save hours or days of waiting for a solution