"Downstage" refers to the part of the stage closest to the audience. "Upstage" indicates the portion of the stage farthest from the audience.
For modern performers, the meaning of these terms might be a bit ambiguous. Consideration of the history of theater construction offers some clarification. The audience seating area in most theaters, from the Renaissance to the late 19th century, was flat. The stage, however, sloped toward the audience so the audience could see the entire stage. Performers moved down an incline, downstage, as they approached the audience and up an incline, upstage, as they moved away from the audience. This type stage is known as a raked stage.
Even though most modern performers work on flat stages, the terms "downstage" and "upstage" are still commonly used.
Some of the historical raked stages, such as the Bolshoi Theatre in Moscow and the Palais Garnier in Paris, are still in use. Rehearsal studios with similar rakes to the stage allow the performers to grow accustomed to working on an incline. Additionally, modern theaters often have features that allow directors to include a rake in the production. These inclines are for aesthetic purposes, making the stage seem larger and giving the impression that the upstage performers are closer to the audience than they actually are.