In "Henry IV Part II," for example, the main plot concerns Henry's growth from "Hal" the prince to "Henry" the king and the reconquest of French territory. A subplot, however, concerns Falstaff's participation in the battles. Falstaff and Henry meet at several points, and Falstaff is a familiar of Henry's, but his plot and Henry's do not mix. Even though they may be thematically connected, they are not connected in action.
A subplot is sometimes referred to as a "B story" or a "C story" and so on, with the "A story," being the main plot. Subplots often involve supporting characters.