Offstage describes something that happens in the area of a theater stage that the audience can't see. An actor who's getting ready for his cue to stride onstage and say, "To be or not to be!" stands offstage.
Offstage is a handy adverb when you're talking about a play. Things that happen offstage might not be visible to the audience, but they're vital to the show: costumers sew on missing buttons offstage, stagehands move parts of the set offstage during scene changes, and actors practice their lines offstage. Offstage works as an adjective or noun as well. You can use it figuratively too, for anything that goes on out of public sight.