The fan belt is a car part which can be bought from Teimo's Shop for 295 mk. The fan belt transmits power from the crankshaft to the alternator and water pump, which supply electrical power and circulate coolant through the engine and radiator, respectively. It is attached to the entire assembly of alternator, crankshaft pulley and water pump pulley.
The tension of the belt can be adjusted by changing the position of the alternator. The fan belt should be installed so that it is one scroll away from squealing for being too loose, as it can snap if it's too tight.