Your Fiat timing belt operates by rotating the camshaft(s) at exactly ½ the speed of the crankshaft while keeping with the precise alignment.  You can have anywhere from one to four camshafts in your engine.  These camshafts open and close the intake and exhaust valves in time with the pistons, which move up and down in their cylinders.  There are two types of engines that require timing belts.  These engines are called interference engines and non-interference engines.  The only difference between these two engines is the distance between the valves and the pistons.  Both of these engine types used to rely on timing chains instead of timing belts, but timing chains are noisier and more expensive to manufacture, therefore, they've been replaced.  If the timing belt slips on an interference engine, it could cause serious damage to your engine.  If it slips on a non-interference engine, it will not cause as much damage, but it will still need to be replaced.