On a perfectly still day, the airspeed is equal to the ground speed. But if the wind is blowing in the same direction that the aircraft is moving, the airspeed will be less than the ground speed. Now suppose we had a day in which the wind was blowing 20 mph towards the west. Since we have defined a positive velocity to be in the direction of the aircraft's motion, a headwind is a negative velocity. The importance of the relative velocity explains why airplanes take off and land on different runways on different days.
Airplanes always try to take off and land into the wind.
Since runways have a fixed length, you want to get airborne as fast as possible on takeoff and stopped as soon as possible on landing. Now mechanical or electronic devices provide the information that is radioed to the cockpit. The relationship between airspeed, wind speed, and ground speed explains why wind tunnel testing is possible and how kites fly. In the wind tunnel, the ground speed is zero. The airspeed is then the negative of the wind speed that is generated in the tunnel.