Chillers are used to keep an aquarium's temperature below ambient temperature.
Chillers usually have a built in temperature controller and switch on when they detect the temperature of the water is higher than what you have set it to.
They are basically a compressor (makes one side cold and the other really hot) and a radiator (cools down the hot side of the compressor by having a fan blow over metal fins).
Chillers need to be well ventilated as they blow out a lot of hot air and function better with cold air. Because of this if you have them with the tank in an unventilated room the room can get extremely hot.
In South Africa we usually need to use a chiller rated for slightly more than our tank size as they are mostly designed for slightly cooler environments.
The radiator and fan should occasionally be cleaned. If they have been running for a few years they may need to be regassed by an aircon technician.