The official name for an aircraft speedometer is an Airspeed Indicator or ASI. Airspeed is a measurement of the plane’s speed relative to the air around it. On the aircraft there is a tube called the pitot tube. The open end of the pitot tube is usually mounted on a wing and faces toward the flow of air. The airspeed indicator actually measures the difference between a static sensor inside the plane (not in the air stream) and a sensor (the pitot) in the air stream.
When the aircraft is standing still, the pressure in each tube is equal and the airspeed indicator shows zero. The rush of air in flight causes a pressure differential between the static tube and the pitot tube. The pressure differential makes the pointer on the airspeed indicator move. An increase of airspeed leads to the pressure at the end of the pitot tube raising.
In turn, the air pressure pushes against a diaphragm that moves a connected mechanical pointer on the face of the indicator (the gauge in the cockpit). The indicator is calibrated to compensate for winds in the air using electronic read-outs from the air and the ground. This system also compensates for altitude and air temperature to make the airspeed measurement accurate.