Contactors are designed to turn on motors, that will draw very high amperage, with a signal input rather than inpu from a human hand. The most common location for a contactor within a home air condtioning system, is in the condenser. It receives a signal from the thermostat to close, which then turns the compressor and the condenser fan motor on. The basic principal is the coil signal creates a magnetnic field that pulls the contacts together. This allows the power to go through the contactor.