When you want to block some of the light hitting the subject, you could use almost anything as a "gobo".

In this example, I am holding a black "reflector" in front of the one lamp that is illuminating the subject. Actually, the gobo is tilted to block the light that is spilling on the wall behind the subject.

You might use a gobo to keep the afternoon sun from lighting one side of your subject too much. Or, you might want to either block (with a gobo) or filter (with a scrim) the light falling on top of a subject's head.