If something happens onstage, it happens on the part of the theater where the actors perform.
Being onstage can make people feel excited or nervous, like when they are in front of a lot of people.
Examples:
The actors are onstage now.
She likes to sing onstage.
The cat is not onstage.
The dog is onstage, but he doesn't know any lines!
There are elephants onstage at the circus!
I want to be onstage and sing like a star.