The timing belt turns the camshafts which in turn, operate the valves in the heads for the engine. The intake valves open to allow the air and fuel mixture to enter the combustion chamber where it is burned to provide power. The exhaust valves open to allow the exhaust mixture to exit the combustion chamber through the exhaust system.
If the timing belt breaks, the valves may contact the pistons, destroying the engine. This type of (valve train) engine is referred to as an interference engine. Not all engines are interference engines. Not all engines have a timing belt.
This is why you replace the timing belt at the miles recommended by your car’s manufacturer. You replace the water pump at the same time, because most of these types of engines have the water pump in the same location as the timing belt. You don’t want to spend the money for the labour to go back in there in a year or two when the water pump fails, so paying the extra for the water pump at the same time is a good investment.
The water pump circulates coolant through the engine and radiator to take the heat of combustion away from the cylinders, heads and engine. Without a water pump, the engine would quickly overheat.