Stage right or left are easy to explain:
- Stage Left: towards the actors left, when standing on stage facing the audience.
- Stage Right: towards the actor's right, when standing on stage facing the audience.
It only gets a little confusing sometimes since it's the opposite of the left and right.
I just learned from my coworker, however, how upstage and downstage got their names (even though I have been working professionally and amateurishly in theater for 15 years!).
First the meanings:
- Upstage: towards the back of the stage, or away from the audience.
- Downstage: towards the front of the stage/audience.
Now the reason:
Stages used to be built with a slight rake (on a ramp) with the front of the stage being lower in height than the back of the stage. This was to help the audience see the actors better, especially when the stage was crowded, because the actors in the back would be on higher ground.
So when the director would tell an actor to go "downstage" on these raked stages, the actor would literally walk down the ramp towards the audience. Even though many stages now are built flat (unfortunately, really), the terms stuck.