New (not rebuilt) a/c compressor assembly. Air conditioning works by compressing a refrigerant gas (it will heat up) which then flows to the condenser radiator on the front of the car to cool and liquefy. When it gets to the evaporator, the liquid expands back into gas, causing a cooling effect. If the compressor has bad internal seals, or the pulley/bearings are bad, the unit will need to be rebuilt or replaced before the AC will work again. Otherwise, the compressor cannot adequately squeeze the refrigerant enough for it to expand and cool the car. Additionally, if a bearing were to go bad or seize, it can cause stress to the rest of the drive belt system.