what is it?

It's a circular device. The dimensions may vary from piece to piece (however, in general, the diameter falls between 10 to  and 20 centimetres). The depth is generally 5 to 8 centimetres. The body hosts the mechanism that makes the device works. Such a mechanism is usually not visible from outside because it is covered by a surface slightly smaller than the body one. Numbers from 1 to 12 are written on the surface. Finally there are two relatively thin pieces of metal which move from the left to the right. When they reach a prefixed pre-set number the hidden mechanism activates a bell.

You could say "pre-fixed" but "pre-set" or "pre-determined" sounds more natural here.