In ice skating, the easiest way of changing direction from forward to backward or vice-versa. It consists of two steps, and the change of direction occurs at the same time as the change of foot. It always goes from an inside edge to another inside edge (on the other foot going the other direction), or from an outside edge to another outside edge.
If you can picture this in your head, or draw it out, you'll see that the two edges will always continue around the same curve. Contrast this to the more difficult choctaw, which is identical except the skater changes edges when they change feet, which leads to an "S" shape.
The most common and easiest mohawk is from the forward inside edge to the back inside edge. If you've ever tried skating backwards you've probably done one without realizing it when you turned around.
The key to making it easy is in the setup: if you pre-rotate your hips and upper body, the change of foot just happens without much effort. The effort is in controlling the rotation going in and coming out, what skaters call the "check".
In ice dance the mohawks are done with an emphasis on control and neatness, and the step will occur with the two feet almost touching at the transition. In freestyle they're commonly done with much wider steps. An advanced freeskater, moving fast, will probably leave the ice (unintentionally) for a few feet when doing a mohawk.
Much like its more difficult cousin, the choctaw, I don't know how the word came to be used in this rather non-Native American-related context.