Backstage is the area of a theater where the actors wait until it's time to walk onstage and play their parts. The audience can't see what's going on backstage.
If you get a backstage pass at a rock concert, it means you'll get to go behind the scenes once the show is over, and possibly meet the musicians in person. Many people who are involved in a show, performance, or play stay backstage the whole time: costume designers, makeup directors, and set painters, for example. The word is also sometimes used in a figurative way to mean "secret" or "behind the scenes."