Shock waves can occur when an aircraft travels at the speed of sound or when a moving source of sound moves at the same speed as sound itself. Compressional wave fronts of the sound waves are "bunched" at the front cause some sort of stretch that when enough speed is accumulated, in this case, the aircraft will break the sound barrier and travel faster than the speed of sound. This is known as a Sonic Boom, and the aircraft involved is known as a supersonic aircraft. I hope you understand this now.
