Like many parts on a vehicle, a timing belt does exactly what it's called. It ensures that the valves and pistons on the engine move in synchronous motion with the camshaft and crankshaft. If your timing belt is broken, you won't be going anywhere in your vehicle.
Moreover, if your timing belt breaks when the engine is running, you could ruin your motor. Since timing belts are made of rubber, after many tens of thousands of miles, the rubber wears thin and can break. To find out when you should replace your timing belt, check your vehicle manufacturer's recommendations.
