The compressor shaft seal sits behind the compressor pulley and clutch assembly. It seals the rotating shaft where it exits the compressor housing. This seal prevents high-pressure refrigerant gas and PAG oil from escaping into the engine bay while keeping dirt out.
This seal is a common source of slow leaks because it experiences constant friction and heat cycles. If you don't run your A/C for months, the seal will dry out and start to leak. Mechanics use ultraviolet dye or electronic leak detectors behind the clutch plate to locate these leaks.