When it comes to technical knowledge of air conditioners, the average persons expertise begins and ends with "is their cold air shooting out of the vent?" Basically, scientists discovered that expanding and contracting gas changed temperature, and some genius engineer applied that to keep us cool. Besides being one of the most common parts of the AC unit that breaks down, the AC compressor is responsible for taking the expanded gas, which is sort of cold, and smashing it down into a compressed liquid, which is really hot, so it can be expanded again, which makes it really cold for a short period of time, which makes your car trips pleasant.