What is a machine pulley on a sewing machine?

The belt usually refers to a rubberized small belt that runs from the motor-shaft to a shaft that moves the needle. It looks like a fat rubber-band. On the older machines, it was clearly visible on the turn wheel. The newer machines have them hidden inside the machine. Depending on your particular machine, you may be able to change them yourself. Some, however, require changing by a repair man.