This means if your timing belt skips or breaks the valve and pistons are no longer in sync and will hit each other causing very extensive damage internally to the point of having to replace the complete engine itself. A timing belt is a VERY IMPORTANT SERVICE.
As you can see in the picture above the timing belt in this case broke and the valves and pistons tried to share the same space at the same time. Note the head of a valve stuck and embedded in the piston.  Good-bye engine, hello big bucks. I would NOT recommend anyone without extensive experience doing this themselves. Even if you succeed in replacing the belt and the timing is off by one tooth the engine will not run properly. If it is off by four or five teeth the damage will have been done.