Most probably the alternator drive belt, we don't have timing belts as the cams are chain driven. To tighten the alternator belt you need to loosen the pivot bolt, one end is held captive by the alternator and the nut is behind the exhaust manifold and almost impossible to get to

then just get a 12mm socket, tighten up the long bolt sticking out of the bracket and re-tighten the pivot bolt. Could also be the power steering pump belt, but I've no idea how to get to that one. Probably more awkward as it's behind the alternator one.
tension bolt is in the middle of the pic, pivot bolt is the one hanging out just up and to the left of it (the nut fell off and it rattled on the bodywork driving me mad till I found it)