In one end, there is a small drawer in which you store the whole nutmeg in. In the other end, there is an opening that the nutmeg you are grating sprinkles out of. You direct that end towards where you want the nutmeg to go.
Fancier models are rotary operated. They are often referred to as “Nutmeg Grinders.”
The goal of either type is to yield freshly-ground nutmeg, whose taste is presumed to be superior to already-ground nutmeg purchased in a tin or jar. The theory breaks down, of course, if the whole nutmeg being used has been hanging around inside the grater since Victoria was on the throne.