An airspeed indicator is a crucial instrument in an aircraft that measures the speed of the aircraft relative to the surrounding air. It displays this speed in knots or miles per hour, helping pilots understand how fast they are flying. The indicator works by comparing the pressure of the air entering a pitot tube with the static air pressure around the aircraft. Airspeed indicators are essential for safe flying, as they help pilots maintain the correct speed for various phases of flight, such as takeoff, cruising, and landing. They are part of the aircraft's instrument panel, which includes other vital instruments like the altimeter and attitude indicator.