The term's original use, from the Latin supernumerarius, meant someone paid to appear on stage in crowd scenes or in the case of opera as non-singing small parts. The word can still be found used for such in theatre and opera. It is the equivalent of "extra" in the motion picture industry. Any established opera company will have a supernumerary core of artists to enhance the opera experience.
Typical supernumerary work
Supernumeraries are usually amateur character artists who train under professional direction to create a believable scene. They almost become part of the props and give a sense of credibility to scenes where crowds, court assistants, lackeys, peasants or a variety of period characters are needed.
While "supers" are often most valued for their ability to fit in their costumes, in some cases they must contribute more to a production. In some cases, supernumeraries also provide specialty skills, for example with stage combat.