A passthrough or wrapper Dll is a Dll that exposes all the functions of another Dll, and when a function is called, it passes the parameters on to that other Dll.

This is usefull when you need to see which calls are being made to a system Dll, since the passthrough Dll can log all calls or even change some parameters if needed. You could for example add parameters to OpenGL calls to make all polygons transparent, which could be usefull for debugging (Or cheating).