When an airplane travels through the air, it produces sound waves. If the plane travels slower than the speed of sound (about 700 mph), sound waves propagate ahead of the plane. If the plane breaks the sound barrier and flies faster than the speed of sound, it produces a sonic boom.
The boom is the “wake” of the plane’s sound waves. All of the sound waves that would have normally propagated ahead of the plane are combined together so at first one hears nothing, then they hear the boom.