Problems with my A/C in my 2007 PT Cruiser?

two weeks or so ago my car started to blow out warm air. and no matter what settings i had it on it wouldn't get colder one bit. So i took it into Firestone yesterday. they said that they couldn't find an obvious leak so they had to use this die. well whatever they ended up doing worked for a second but than it went right back to blowing warm air. So now they call me and tell me that they ordered valves and they are going to put them in and see if that is the problem. Now what are valves and what do they have to do with the AC? this is my first car and first time dealing with an auto shop and i know basically nothing about cars. i don't want get screwed over. or end up spending a crap load of money. So please any advise will help..