What is PCH and bAdaptiveUnityDisablesPCH?

During compile this is in the output window:

2>[Adaptive unity build] Disabling PCH for excluded files. Set bAdaptiveUnityDisablesPCH to false in BuildConfiguration.xml to change this behavior.
2>[Adaptive unity build] Excluded from Rover unity file: <list of some seemly random files from my project>

Can someone provide a summary of what PCH is and what is happening here?

PCH stands for Pre-Compiled Header.

It’s a file that is compiled once, and then never built again (as long as the files used to create the PCH aren’t modified). If you modify some file that is contained within that PCH, then you can see Unreal disable the PCH for that module/etc(causing all files that touch that PCH to be recompiled manually).