Is this source file only a header file.

A property on a source file that indicates if the source file is a header file with no associated implementation. This is set automatically based on the file extension and is used by CMake to determine if certain dependency information should be computed.

By setting this property to ON, you can disable compilation of the given source file, even if it should be compiled because it is part of the library's/executable's sources.

This is useful if you have some source files which you somehow pre-process, and then add these pre-processed sources via add_library() or add_executable(). Normally, in IDE, there would be no reference of the original sources, only of these pre-processed sources. So by setting this property for all the original source files to ON, and then either calling add_library() or add_executable() while passing both the pre-processed sources and the original sources, or by using target_sources() to add original source files will do exactly what would one expect, i.e. the original source files would be visible in IDE, and will not be built.