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What is a compressor used for?

It is used for compressing air. The compressed air can be used for filling tires or powering pneumatic tools. A compressor under the hood of a commercial truck usually compresses air for the air brake system.

If you are talking about the compressor in your car, then it is probably the air conditioning compressor. It pumps freon around and enabling the cooling of the air inside your car.
Some cars also have air suspensions, and these use an air compressor to provide air pressure to adjust the ride height of the car.

So it has nothing to do with installing a head gasket?

Oh, a valve spring compressor tool. Why didn’t you say so?

I wonder if the mechanic was talking about “compression” as opposed to compressor. As the piston comes up in the cylinder, the fuel mixture is compressed, which means that it is packed into a tighter space. As this mixture is compressed,it is ignited by a spark which forces the piston back down the cylinder. If the head gasket is defective, the piston, as it rises, is not compressing the mixture and the engine won’t perform properly.

Picture it like a balloon half inflated with air. If you squeeze the balloon, at one end, the other end of the balloon inflates. You have forced the air into a smaller space. However, if you punch a hole in the end of the balloon, the air will leak out and won’t fill the space. A defective headgasket has the same effect. The piston can’t compress the fuel/air mixture because it leaks out.

Please describe your situation more completely. Include make/model/engine in car, problem, diagnosis, etc.

Because mechanic only said work was delayed a/c their compressor went out, which was the reason for my original question. They have now had my car for a week to replace a head gasket. Good thing they are paying for the rental.

Their air compressor failed so their air tools don’t work.

sounds to me the mechanic was refering to his AIR COMPRESSOR. Hence the delay, “Because mechanic only said work was delayed a/c their compressor went out”