A Mach front is one of the lesser-known effects of nuclear weapons. It is found only after a ground burst or low-altitude airburst. When a nuclear weapon detonates, it sends out a shock wave. This shock wave expands outward from the center of the blast, and the lower half of it hits the ground (in theory, at least). As this happens, the shock wave is reflected back upwards. If it struck the ground at an angle (as it does unless it's directly under the blast) this reflected shockwave also begins to propagate outwards. There is an area (a constantly changing and moving one) where these two shockwaves meet and constructively interfere. This union manifests as an extremely high-speed wall of overpressure. Because the shock waves meet at an angle, the standing wave of their intersection can propagate faster than sound travels, in much the same way the 'point of intersection' of a scissors can move extremely fast as you snip. Hence, it's known as a Mach front to indicate its potential supersonic speed and high pressure.
An excellent photo of an atomic explosion which clearly shows the shock waves (both direct and reflecting) can be found here. The Mach front hasn't yet fully formed, but will be the region where the two waves intersect. The photo was run with a New York Times article which can be found here, but requires registering for the site.