Accessory belts is the new term for what use to be called “fan belts”. These are the reinforced rubber belts that drive all the components on the engine like the Alternator, Air Conditioning Compressor, Power Steering Pump, Water Pump, Supercharger, and so forth.
Their function is to keep the components rotating smoothly and efficiently by gripping their respective pulleys with enough force to maintain a smooth rotation. When this smooth rotation is not happening like on an alternator, the alternator starts introducing high frequency voltage spikes into the electrical system. These voltage spikes if they become severe enough can cause driveability problems including false check engine lights, vehicles in ‘limp in mode’ and more. It could also confuse the PCM through uncontrolled changing load factors.