The cam belt is driven by the crankshaft which is turned by the action of the pistons moving up and down.
As it turns, it turns the camshaft, which controls the inlet and exhaust valves movements, opening and closing them at exactly the right time.
The valves open down into the cylinder and do so when the piston is down.
If the belt should snap, there is a very good chance that the upwardly moblile pistons will meet the open valves resulting in metal carnage. Very expensive to repair.