Figure to ground means that a subject or idea (figure) is clearly defined against a background. This can be achieved through technical means, ie depth of field, or compositional means. My preferred method is to make sure that everything in my photographs fit neatly in place. Unless elements are specifically interacting then they do not need to touch. No lampposts coming out of heads in portraits, no walls going through bodies.
Everything where it needs to be, like pieces of a puzzle.