Lead Right in principle can be done by any facing couples, but in practice it is almost always done from a squared set, by either the heads or the sides. It means that each couple moves forward while turning, as a unit, 1/4 (90 degrees) to the right, sliding past each other so that the two couples end back to back. From a squared set, the result will be that the couples who did the Lead Right end up facing the couples who didn't. For example, after a Heads Lead Right, the head couples will be facing the side couples.
Lead Left is the same, except that the couples turn to the left.