The body of the caliper slides back and forth (side to side) on two pins mounted in bushings that end in rubber bellows covering the exposed parts of the pins. When these lose their lubricant or corrode, they seize up and the pads and rotors wear very quickly. Often you can merely remove the fixing bolts, pull the pins out, clean and polish them, and lube with synthetic brake lube. On older calipers and in salty areas, the pins are usually so corroded that you need either a rebuilt kit (available from Volvo) or a rebuilt caliper.