WINMD files contain API definitions. APIs allow programs to access WinRT components. Third-party developers create custom WinRT APIs. They must create WINMD files too. These files let Windows and programs use the APIs correctly.
The file name matches the assembly name. The assembly is inside the WINMD. The file system is case insensitive. So capitalization may differ from the assembly name.
WINMD files work on Windows. Visual Studio uses them the most. The Microsoft website has WINMD details. And Visual Studio info.
WINMDs use the CLR format. But they contain different data than assemblies. WinRT APIs access Windows Runtime Components. WINMDs accompany these.
Visual Studio
ILSpy
Windows SDK