When an airplane goes supersonic, it creates a shock wave in front of it. The shock wave slows the air down to subsonic speeds, so the airplane sort of creates it's own cocoon of subsonic air to fly in.
When the air comes through the shock wave, it's speed reduces to below supersonic, and the pressure increases. The highest pressure is found behind the shock wave, in a stagnation point, a point where the speed is reduced to zero, like on the leading edge. The lowest pressure is usually found before the shock wave, this is the normal atmospheric pressure.
There are 3 types of shock waves:
- the normal shock wave:
They have nice tables where you can see the speed, pressure and density behind the wave.
- Oblique shock waves:
These shock waves are called strong when the mach number after the wave is below 1, and weak when the speed after the wave is still supersonic.
NASA has some nice programs to show how different types of shock waves are formed and how they interact, you can find them here: