If the plane is traveling slower then the speed of sound, then sound waves can spread out ahead of the plane. If it breaks the sound barrier and flies faster than the speed of sound, it produces a sonic boom when it flies past. The boom is the wake of the plane's sound waves. All the sound waves that would have normally spread out ahead of the plane are combined together, and you hear the boom. When you're on the shore of the ocean and a boat zooms past, at first there is no disturbance in the water, but shortly after, a large wave from the wake crashes up to the shore. When a plane flies past at supersonic speeds, the same thing happens. Instead of the large wake wave, you'll hear a sonic boom. Another way to think of sonic booms is to imagine all the molecules that make up our air. When planes fly through the air at moderate speeds, the molecules have time to move aside to let the plane through.