My freezer isn't working and I've had a repair guy in to look at it. He said the compressor needs replacing! The light is on in the fridge and there is a humming sound going on but still no cold air or ice. Could some one please tell me what a compressor does?

I've just spent the last hour or so going round in circles with various links from google. I don't see why I have to pay £13.00 just to find out that piece of info. Would some kind person please enlighten me?