Just as a boat moving through the water forms a series of V-shaped waves, airplanes create conical (cone-shaped) waves as they fly through the air. The waves that the airplane produces are waves of compressed air. When an aircraft reaches the speed of sound, Mach 1, the plane’s pressure waves that move at the speed of sound overlap each other, creating a shock wave. The shock wave creates one single, loud sonic boom heard by observers on the ground. When the plane travels slower than the speed of sound, the sound waves do not overlap and instead of hearing a sonic boom, observers simply hear the delayed sound of the plane.
You can think of sound waves as being similar to water waves, emanating from a central source and spreading out in a regular pattern unless they are interfered with.