What is the meaning of "-WaitMutex" ?

When compiling with UnrealBuildTool.exe I often come accross -WaitMutex,
What does it do?

Mutex is a special synchronization primitive. Most often it is used to avoid multiaccess to some commonly used resource that should be used in monopolic manner. In this case, UnrealBuildTool.exe can build only one proj at the moment and if you try to use it with another project at the same time it can throw this exception.

Thank you! Could you make this an answer so that I can validate it?

Mutex is a special synchronization primitive. Most often it is used to avoid multiaccess to some commonly used resource that should be used in monopolic manner. In this case, UnrealBuildTool.exe can build only one proj at the moment and if you try to use it with another project at the same time it can throw this exception.