Hey Fans!
I get this question asked a lot. What is a Timing Belt, and why do I have to change it?
Simply put, the timing belt is a toothed belt that keeps the engine in sync, or as its name mentions, time. There are essentially 2 halves of an engine, an upper half and a lower half.
The bottom side of the engine is the crankshaft, it is known as the rotating assembly because the pistons are connected to the crankshaft. 
As the explosions happen from the fuel ignition in the cylinder, the pistons are forced downward causing the crankshaft to rotate. The crankshaft has a toothed gear on the front that the timing belt rides on. This rotation causes the belt to move and spin the other half of the engine that contains the camshaft, or multiple camshafts. They also have a toothed gear attached. This camshaft pushes on the valves against the springs to make them open. This lets air and fuel in on the intake side, and also lets burned exhaust gases out through the exhaust side. As you may imagine, those things must occur at the right “TIME”.
When the belt, which is made out of a rubber material, ages, the teeth start to crack on the inside and can shear off or “break”. 
This break in the belt will allow the lower end to spin, but not spin the belt thus causing the upper end to stop moving.
There are more than likely a valve or two that are still open when the piston comes up and with an interference engine, the piston will hit and bend the valve. This is engine failure and will require engine disassembly. Replacing the timing belt is a far less costly service and will prevent the engine repairs from being needed.
The timing belt also drives the water pump in a lot of engines, it is a good idea to replace preventively at the same time. You should also replace the tensioner as well. Any time you can access harder to reach areas, it is better to replace some things preventively.
I hope you have a little bit better understanding of the importance of the timing belt in your engine.
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