Normally interferograms have bands, or interference fringes (as in the above figure). Why do these occur? The reason is that the interferometer is deliberately misaligned slightly to make them appear. When one beam is tilted relative to the other, you get straight bands. The bands make it easy to see any errors, since a warped wavefront bends the fringes.

These fringes also make it possible to easily measure the shape of the wavefront. Each dark fringe is exactly one wave away from the previous one. The more tilted the wavefront, the more fringes you get. You can easily measure the tilt by counting the number of bands across the aperture. If you see five bands across the optic, the tilt is five waves. So now we can measure the tilt of a wavefront easily. What happens if one of the wavefronts isn't perfectly flat? Now the fringes start to bend, revealing the shape of the wavefront (above figure). If we measure the positions of the fringes everywhere, it is possible to figure out the exact shape of the wavefront. Actually figuring out what the shape is from those measurements requires some fancy mathematics. That's what Quick Fringe does for you.