It keeps crank and cam in time. Not every engine has one: some use a chain.
Teaching inline-four schematic. Not a specific car. Cam belt
On an interference engine a snap can bend valves. This schematic is a dry belt under a cover — not a chain, not a wet belt.
Under the timing cover, at the end of the engine.
208 1.6 HDi timing belt (model page, FR)
The interval is the manufacturer’s for THIS engine. A 208 1.6 HDi figure is not a petrol engine’s, nor a wet belt’s.