This is the electronic thermostat which has the primary role of regulating AC operation based on the temperature setting on the control panel and the temperature reading from the frost sensor (clamped on the evaporator tubing).

When the values from the two sources are equal then this device stops operation of the compressor and condenser fans; as temperature on the evaporator pipe increases then the thermostat triggers compressor and condenser fans and the system cycles again. This process of A/C operation is called cycle on/cycle off.