A fundamental principle of Gestalt psychological theory
that organizes perception
into an object
) that stands out against a background
(duh, the ground
The classic proof of this is the "Vase or Faces" illustration. Depending on the viewer's focus the contrast between black and white can be perceived as either figure or ground. If you concentrate on the white area as the object then you will perceive a white vase (which actually looks a lot more like a goblet) over a black background. If you focus on the black portion as the figure you will see two faces in profile with a dividing field of white between them. There are several other reversible figures but this one is probably the most famous. I first saw it in junior high school if not earlier.
This relationship is not limited to visual perception. Being able to focus on the vocals or guitar in music or to sort out an itch for immediate attention is also indicative of this method. Many psychologists believe that we are born with this ability and that it does not have to be learned.