Find out the role of all the C# stuff

I use Unreal Editor and code in C++, but there is a lot of C# 'glue' in Unreal. I need to find out what it is used for, and what role it plays in the integration between Unreal Engine, Projects, and the C++ IDE (CLion in my case).

What kind of changes require me to run 'GenerateProjectFiles.sh' again
What kind of changes require me to do 'File -> Refresh CLion project' in Unreal Editor