Airspeed is a measurement of the plane's speed relative to the air around it. The pitot (pronounced pee-toe) static tube system is an ingenious device used by airplanes and boats for measuring forward speed.
The device is really a differential pressure gauge and was invented by Henri Pitot'in 1732.
Heres how it works...==
... The open end of the pitot tube, usually mounted on a wing, faces toward the flow of air or water. The air speed indicator actually measures the difference between astatic sensor not in the air stream and a sensor (pitot tube) in the air stream.
An increase in forward speed raises the pressure at the end of the pitot tube. In turn, the air pressure pushes against a flexible diaphragm that moves a connected mechanical pointer on the face of the indicator. The indicator is calibrated to compensate for winds in the air or the speed of the opposing current in the water.
In airplanes, electronics also compensate foraltitude and air temperature to make the air speed measurement accurate.
An example of an air pressure gauge is a tire pressuregauge