An airspeed indicator is an essential instrument in an aircraft that measures the speed of the aircraft relative to the surrounding air. It displays the speed in knots or miles per hour, helping pilots understand how fast they are flying. This information is crucial for safe takeoff, landing, and maintaining proper flight performance. The airspeed indicator works by comparing the pressure of the air entering a tube on the aircraft's exterior to the static air pressure. This difference in pressure allows the instrument to calculate and display the aircraft's speed, ensuring pilots can make informed decisions during flight.