This folder contains generated code and debugging symbols. It is required in order to debug .dmp files generated from crashes. As the name suggests, it should not be included into the steam build (that is, people who download your game shouldn’t get these files), however, you should store them somewhere that you can access them later when you need them.
Keep in mind that once you delete them, making another build will not produce identical debugging symbols and will cause you to be unable to debug any .dmp files from previous builds should you ever want to.
The folder contains debugging symbols and generated C++ source code. When you attach a debugger to a game, your debugger will ask you to browse to the symbols and/or source code.
We don’t have much on our docs site about debugging native C++ code. There’s Unity - Manual: Windows debugging that provides basic information about Unity’s own symbol files for Windows but it predates IL2CPP scripting backend so it doesn’t mention this folder (it probably should, I’ll make a note of that). Generally, we would refer you to documentation of each individual debugger as it’s not practical for us to duplicate it. For instance: