A timing belt, timing chain or cam belt is the part of your engine that synchronizes the rotation of the crankshaft and the camshaft(s) ensuring that the engine’s valves open and close at precisely timed intervals.
Your engine’s valves operate in such a way as to allow fuel and air into and out of the engine’s combustion chamber as part of the combustion process. Where combustion (the normal ‘explosion’ of fuel and air being ignited in your engine) occurs – the valves push the pistons in your engine up and down, providing the power to drive your car forwards.
If the valves and pistons do not open and close at the right times due to a timing belt issue, they can collide with each other. As the pistons are larger, where this happens they will damage the valves, potentially locking up your engine.