A backdrop is the scenery that hangs behind the actors in a play. You might help to paint the backdrop for your school's production of "The Sound of Music."
The backdrop is an important part of a theater production, since it provides a setting for a play or sets a mood for a musical performance. If you're watching a local version of "Oliver!" the backdrop might be designed to look like a London street scene, for example. The noun backdrop comes from US theater slang, from a combination of back and drop around 1913.