Single-stage means no clearcoat; the color coat is the final application of paint.
2-stage refers to a basecoat (color) and clear overcoat; Basecoats for 2-stage systems have no gloss, they're basically satin, and the gloss and protection are provided by the clear topcoat.
Single-stage paints are not formulated to be cleared over; it can be done, but there may be compatibility issues that won't allow the two chemicals to properly bond.