Learn Your Directions
Acting on stage requires the knowledge of a new kind of navigation. When you are on stage, directions are oriented to you as the actor. (Conversely, House Left (HL) and House Right (HR) refer to directions oriented to the audience.) In a script, you’ll see blocking that may read any of the following: SL, SR,DSL, DSR, USL, USD. Here’s what they mean:
SL: Stage Left – Your left when you are standing on stage.
SR: Stage Right – Your right on stage.
DSL: Downstage Left – Moving toward the audience, to your left.
DSR: Downstage Right – Moving toward audience to your right.
USL: Upstage Left – Moving to the back of the stage, left.
USR: Upstage Right – Moving to the back of the stage, right.
Downstage and Upstage refer to the old days of theater -- think Shakespeare -- when stages were "raked," or sloped. When you moved toward the audience you were literally moving in a downward direction and upward when you moved to the back of the stage.
http://www.sft.edu/tips/stage-acting.html