A timing belt keeps the crankshaft and the camshafts turning in the right relationship, so the valves open and close at the right moment relative to the pistons. It is a reinforced rubber belt, and unlike most parts on your car it gets replaced on a schedule rather than when it starts acting up. That is the whole idea behind it. There is no warning noise, no dashboard light, and no inspection that can tell you a belt has five thousand good miles left in it.
The interval comes from whoever built your engine, and it varies quite a bit between manufacturers, so your owner's manual or factory maintenance schedule is the authority here, not a rule of thumb someone repeated at a car show. Age counts as well as mileage. Rubber cures over time, and a belt on a low mileage car that has sat through fifteen Florida summers is not in the shape the odometer suggests. If you bought the vehicle used and nobody can document that the belt was done, that is worth settling.
Plenty of engines are what is called an interference design, meaning the valves and the pistons pass through the same space at different moments. On those engines, a belt that breaks or jumps teeth lets them meet, and the job turns into internal engine work instead of a belt. Not every engine is built that way. We will look yours up and tell you plainly which kind you have, without dressing it up.
The belt is rarely replaced by itself, and not because we enjoy selling parts. The tensioner and idler pulleys spin on bearings that have covered the same miles as the belt, and a seized pulley destroys a new belt just as thoroughly as an old belt fails. On many engines the water pump is driven by the timing belt and lives behind the same covers, so replacing it while everything is already apart keeps you from paying that labor twice. We cover water pump work in more depth on its own page.