Figure-ground refers to the shapes, space or forms within a composition. In simplest terms the figure is what you notice and the ground is everything else.
The figure also known as the positive space, refers to the image(s) that are visually dominant on the ground.
The ground, sometimes referred to as the ‘negative’ space or the background, is the surrounding area that the figure is placed upon.
An image can be figure dominant or ground dominant.
The mind tends to seek distinctions between the figure (the subject of the design or image) and the ground (the background).
A designer typically wants the figure to demand attention; the ground should support the figure and not distract the viewer.
Distinctions between figure and ground can be accomplished in a number of ways:
Contrast of color
Blurred or out of focus background
Placement of the figure in the image
Magnifying the figure so that the ground is virtually non-existent
Minimizing the figure so that the figure appears to be isolated or insignificant. (Source)