The airspeed indicator is sensitive differential pressure gauge, which displays the pressure differential between dynamic air pressure from the pitot tube and static, or ambient, air pressure.
The pitot tube receives total air pressure through the opening in the front of the tube. Total air pressure is the total of dynamic and static pressures.
Since the pitot tube is open to the atmosphere, it always receives static pressure. When air moves over the pitot tube, dynamic pressure is also generated by that movement.
This total pressure is routed to one side of the airspeed indicators pressure sensing mechanism. Static air pressure from the static port is routed to the other side. The static pressure from the static line cancels out the static pressure sensed by the pitot tube. This leaves only dynamic pressure to be displayed on the airspeed indicator.
A hole in the back of the pitot tube allows moisture to drain away when the aircraft flies through precipitation.
Types of Airspeed
Indicated Airspeed: read directly from the instrument. IAS is uncorrected for atmospheric density, installation error, or instrument error. Takeoff and landing speeds listed in the AFM or POH are IAS and do not normally vary with altitude or temperature.
Calibrated Airspeed: IAS corrected for installation and instrument error.
True Airspeed: CAS corrected for altitude and nonstandard temperature.
Mach Speed: the ratio of the current true airspeed to the speed of sound.