Some of us remember a time when jets flying overhead routinely created a sonic boom: a thunderous boom which usually came in pairs whenever a jet plane flying overhead was flying faster than the speed of sound. A plane pushes up pressure waves out in front of itself as it flies, much like a boat does on water. These pressure waves travel at the speed of sound — around 700 mph depending on the altitude and air density.
If the plane speed reaches the speed of sound, the pressure waves cannot get out of the way so they build up and generate a shock wave which can be heard at a considerable distance from the plane.
A second shock wave is generated by the plane's tail as the air pressure returns to normal. Jets are rarely allowed to create sonic booms over populated areas anymore because the booms were so disruptive.