Originally from the theatre. A backdrop is a painted cloth in front of which the actors perform.

Now a backdrop is taken less literally to mean the background or setting to many things. For example a political backdrop is the history of a topic in front of which a current affair is played out. "Shares were a touch firmer in opening trade on Tuesday, despite the cloudy backdrop provided by global markets. " (from BBC news online.)

A backdrop can also used to describe the background picture displayed on a computer desktop.