When a “normal” airplane travels through the air, a set of pressure waves that travel away from the airplane at the speed of sound are created ahead of and behind it, sort of how the wake of a boat forms. As the airplane travels faster and nears the speed of sound the waves are forced together and at the speed of sound (Mach 1.0) they merge together to form a shock wave. Typically, in smooth flight the shock wave starts at the nose of the aircraft and ends at its tail.
To an observer on the ground, two sonic booms are typically heard after the aircraft passes overhead—the first boom occurs when the initial pressure shock wave hits the observer, followed almost immediately by a second boom when the pressure returns to normal as the shock wave passes.
The size or intensity of the shock wave and thus the sonic booms depends the amount of air accelerated by the shock wave, which is largely determined by the size and shape of the aircraft. Thus, the Concorde produced significantly more intense sonic booms than would a smaller military fighter aircraft.