Regnant is an adjective meaning "reigning." Most often it is contrasted with consort. For most of history, at least in the Western world, a Queen did not reign; the woman who married the King was the "Queen Consort." Elizabeth I, however, was a "Queen Regnant."
In the SCA the main use of the term is to draw a distinction between a Crown Prince/Princess, who is going to be King/Queen soon, but who does not reign anywhere, and a Prince/Princess Regnant, who rules over a Principality.