A timing belt, timing chain or cambelt is a part of an internal combustion engine that synchronizes the rotation of the crankshaft and the camshaft(s) so that the engine's valves open and close at the proper times during each cylinder's intake and exhaust strokes.
Failure to replace the belt can result in complete breakdown or catastrophic engine failure, especially in interference engines.
The usual failure modes of timing belts are either stripped teeth or delamination and unraveling of the fiber cores.
Correct belt tension is critical - too loose and the belt will whip, too tight and it will whine and put excess strain on the bearings of the cogs. In either case belt life will be drastically shortened.