Your timing belt choreographs the timing of your combustion process. Your pistons travel up and down in the cylinder. Intake valves open at the right time to let in air and fuel, they close at the right time to allow the fuel to burn and then the exhaust valves open at the right time to let out the exhaust.
All this happens thousands of times a minute and it's your timing belt that makes sure the valves are opening and closing at precisely the right time. If the timing is off, your engine won't run. And that's the best case.
The worst case is that a valve is opening at the wrong time and collides with the piston. The result is bent valves and maybe even more damage to the cylinder head. Repairs can run several thousand dollars.
Timing Belt Repair & Maintenance