The diagram shows the Doppler effect. Waves are being emitted by the biplane - a moving source. The waves are 'squashed' in the front of the plane and
spread out behind the plane. The speeds of the planes shown in this animation are less than the speed of the waves.
You first see a jet plane flying close to the speed of sound and when this has disappeared a biplane flies across the screen much more slowly. The waves from the biplane stop when the plane has moved more then half way across the screen.