Supporting leg is a classical ballet term that describes a dancer’s leg that is supporting their whole body while the other leg is free to do another step. A dancer’s supporting leg is often called a “standing leg.”
For example, if a dancer is in retire with their right leg during a pirouette, their supporting leg is their left leg, or the one they’re “turning on.” Or, a dancer may perform a developpe a la seconde, with their left leg, while the right is their supporting leg.
Sometimes both feet are still connected to the floor during a step, such as a tendu, that has a supporting leg, which is why you can’t always just say, “the leg that connected to the floor!”
A strong supporting leg is quite easily one of the most important aspects of solid classical ballet technique (and all dance forms!).
It is a basic part of technique that is sometimes overlooked into intermediate and advanced levels in schools because a student is more concerned about their “working leg.” Because of this, sometimes you may see a dancer unknowingly let their supporting leg bend a little (or a lot!) when trying to do high grande battments, ronde de jambe en l’air, pirouettes, or even basic steps like tendus.
Many advanced dancers can imagine themselves being “stacked and aligned” over their supporting leg. Much like a structure that needs a strong base to stay standing during a storm, a dancer needs a strong supporting leg to properly do many classical ballet steps for technical and aesthetic reasons.