The purpose of the car's air conditioning system is to conditionsthe air, which means whilst cooling the air in the car down, it also helps to ensure there isn't too much moisture in the car, so it stops the air getting too humid. The car's air conditioning unit also cleans the air in the car by filtering the dust and other particles from the air inside the car.
The aircon switch is what you press to activate the car's air conditioning unit. It is on the aircon unit, which is usually situated as part of the controls on the car's dashboard.
When you press the aircon switch on the aircon, the air conditioning will not activate until the aircon controller unit tells it to. The car's main Electronic Control Unit (ECU) decides, from information provided by specialised sensors, the right time for the car's air conditioning to start working. The car's ECU will monitor various factors like the temperature of the car, the speed of the engine and the temperature of the engine coolant to check not only if the air conditioning is needed in the car but also if it is safe to start the aircon compressor, which sets the whole process in motion. At the right time the ECU will tell the aircon controller unit, which is set to operate the heating and air conditioning system, to start cooling the car and the process will begin.
This part, being used lots, will wear out at some point and not work so well, or you may experience the aircon switch jamming completely and are unable to operate it and will need to replace it with a new aircon switch.