The airspeed indicator measures the so-called "indicated air speed" (IAS) by measuring the difference between total and static pressures of the airsteam. The pressures are sensed by a Pitot tube and fed to the instrument by a hose.
The pressure difference distors a diaphragm with carefuly defined and stabilised elastic characteristics. The deformation through a linkage and geartrain causes deflection of the pointer.
Color markings on the instrument's dial can identify the operating ranges of an aircraft.