Upstage is farthest away from the audience. Downstage is closest to the audience. Stage right is on your right when you are facing the audience. Stage left is your left when you are facing the audience. These directions form the basis of dividing the stage into a grid of nine equal parts.
Abbreviated as CS, this is the exact center of the stage. This is often where the best acoustics are found.
Down right (DR) refers to the corner of the stage closest to the audience and on your right as you face the audience. Down left (DL) refers to the corresponding corner on the left side of the stage as you face the audience. Down center (DC) refers to the center of the stage, close to the audience.
Up right (UR) refers to the corner of the stage farthest from the audience and on your right as you face the audience. Up left (UL) refers to the corresponding corner on the left side of the stage. Up center (UC) refers to the center of the stage, farthest away from the audience.
Right center (RC) is on your right as you face the audience, and about halfway from the audience to the back of the stage. Left center (LC) is on your left as you face the audience, and about halfway from the audience to the back of the stage.
Cross (X) means to move. Therefore, the notation XRC means to move right center. OS stands for offstage. A director might tell an actor to cross left and exit upstage. The actor would then write the notation: XUL OS.