Here’s the explanation and example from WordPress (text and photo by Pick):
“In the Background: The places that we pass through day after day, or even once in a lifetime, leave in their small way, echoes and traces of themselves upon us. But so often when taking self portraits or pictures of friends, the places themselves become a soft blurred mush of indistinct semi-nothingness, the limelight stolen by our smiling faces. In today’s challenge, let’s turn the tables.
“Take a picture of yourself or someone else as a shadow, a reflection, or a lesser part of a scene, making the background, or — as in the example above — the foreground, the center of attention.”