A belay device is used by climbers as a break. Their main purpose is to allow the rope to be locked off with minimal effort to arrest a climber’s fall. The belay devices mechanism is based on friction that is created when the rope slides through it. There are many kinds of belay devices, some of which may additionally be used as descenders for controlled descent on a rope, as in abseiling or rappelling. Passive belay devices rely on the climbers brake hand and a carabiner to lock off the rope.
Here you can see a great video by eHowSports on how to use the ATC belay device.