Float Switches are what hangs down on your pump in your pump tank. As the water level rises the float that controls your pumps on off function rises as well . At a certain water height the switch will be activated normally 45 degrees turning the pump On. As the water level gets lower so does the float turning your pump off . There are 2 types of pump float switches for aerobic systems . "Mechanical" and "Mercury Activated". "Mercury Activated" is often cheaper but " Mechanical" pump float switches are more reliable and last longer.