Those are instructions to gcc to link in the libraries /usr/X11R6/lib/libXi.so and libXmu.so at link-time. I found I needed them for my little glut app to build on Redhat 9.
If your glut app builds and runs without explicitly linking to them – great. (Though, in that case, I don’t know what’s going on – maybe libglut.so has those libs statically linked into it?)
the -l just says search for and link to these libraries when linking. In this case you want to search the Xi and Xmu libraries and link to whatever calls your using in your program. Those libraries are just Xlib libraries I believe.