An accessory belt (also called a drive belt or serpentine belt) transfers rotational power from the engine crankshaft to run accessories. As the engine runs, this long, ribbed rubber belt loop spins the alternator to charge the battery, the water pump to cool the engine, the power steering pump, and the A/C compressor. Modern engines usually rely on a single belt to run everything.
Belts suffer from dry rot due to engine heat. They develop small cracks along the internal ribs. If you ignore these cracks, the belt will eventually snap, disabling all accessories. Your steering will get heavy, the battery will stop charging, and the water pump will stop turning, causing the engine to overheat within minutes.