The eye differentiates an object form its surrounding area. A form, silhouette, or shape is naturally perceived as figure(object), while the surrounding area is perceived as ground (background).
BASIC FIGURE-GROUND: All of the text on this page is Figure and the background white is Ground.
Balancing figure and ground can make the perceived image more clear. Using unusual figure/ground relationships can add interest and subtlety to an image.
-paraphrased from http://graphicdesign.spokanefalls.edu/tutorials/process/gestaltprinciples/gestaltprinc.htm
The tree is is the Figure in this photograph, while the sky is the Ground.
Consider what is Figure and what is Ground in this typographic layout.
"Figure ground ambiguity is the visual illusion with two alternate viewpoints. This is similar to figure ground reversal, but the alternate image creates a totally different perception. In this version of figure ground, a pair of objects share a similar edge. This illusion is created by the inversion of figure and ground. A well-renowned figure example is Rubin’s Vase, developed by psychologist Edgar Rubin. In this image the black positive space forms two faces that appear to be ready to kiss, and the inverse negative space forms a vase. Visually the concentration on either the white or the black makes the illusion alternate between the vase and the faces." - http://605.wikispaces.com/Figure+Ground
The photo below plays with ambiguous Figure-Ground, showing both a human skull and/or 2 astronauts in front of a planet.