For a cmake project, compile_commands.json contains exact command line compile command for each source file in the project. The package analyses compile_commands.json and provides easy access to compiler command, compile args, compile includes, etc. Other tools like flycheck-cmake or auto-complete-clang then can use this lib to support cmake projects easily.
compile_commands.json can be generated via below cmake command:
cmake -DCMAKE_EXPORT_COMPILE_COMMANDS=1
Just need to add build directory containing compile_commands.json to cmake-compile-commands-build-directories:
(setq cmake-compile-commands-build-directories '("/project/build"))