In Disney theme parks it is any area to which guests have access. Cast members must be on their best behavior onstage and must display good show (certain standards set forth to them by the company).
Certain things not allowed onstage: eating, drinking, smoking, talking on the phone, talking about one's personal life, using guest restrooms, being unfriendly.
See also: backstage, cast member, guest, Fastpass, lead
I need to get off the phone. I'm about to walk onstage.