Plain English Meaning
Small switches on the landing gear that tell the airplane whether it is sitting on the ground or flying. They flip one way when the wheels are pressed down by the airplane's weight, and the other way when the airplane lifts off and the gear hangs free.
Full Definition
Electrical switches mounted on the landing gear struts that sense whether the airplane is on the ground or in flight. When the gear is compressed by the airplane's weight, the switch is in the 'ground' position; when the strut extends after liftoff, the switch moves to the 'air' position. This signal is used by other aircraft systems to enable or disable certain functions depending on whether the airplane is on the ground or airborne.