It might help to explain the mechanics behind elliptical fitness equipment. It is like having both a ski machine and a stair-stepper. Your feet slip into pedals and move the roller or a fly wheel track in an extended oval shape movement, or ellipse.
Why is that a good idea? Reason number one, it is a low impact exercise machine. You get the same, if not a better, workout as if you were on a treadmill, but your joints take less of a beating in the process. Your knees, ankles...