okay, a simple example:
You are sitting at a table. There is a piece of cloth on the table and a glass of water sitting on this cloth.
You are looking at the glass of water. It is the figure on the ground of the cloth.
Now you want to drink from that glass. But unfortunately for some reason you can stretch out your hands just a little, not enough for reaching the glass.
Now the following might happen: You realize that the cloth is lying on the table and by pulling the cloth you can move the glass within reach of your hands.
What has happened here? To find this solution for your problem you had to be able to see the cloth (which was and still is ground for the glass) as figure on the ground of the table. This ability to change in figure / ground formation was necessary for the solution of your problem.
Small children are at first not capable of this transformation. Adults sometimes lack this ability too in specific situations or because they are more or less disturbed. In this sense one can say that the ability for this figure / ground transformation may be a sign for healthy functioning.
I hope it is obvious that what I have demonstrated for such a simple case is important also for other, much more difficult problems man has to solve in his life. But not every problem can be solved by figure / ground transformations. Many problems demand other transformations and re-organizations of one's perceptual and behavioral field like re-centering, re-grouping, finding an adequate closure and so on. And the ability to do such re-organizations can be developed or hampered in many ways. One could say that what happens in psychotherapy is more or less the search for and actual progress in such adequate re-organizations, a process which in most cases demands the involvement of the whole (feeling, thinking, acting) person.
I hope this answers your question. If you would like to see these ideas applied to a practical psychopathological case have a look at Gestalt psychologist's Erwin Levy's article on formal schizophrenic thought disorders in the GESTALT ARCHIVE (at http://rdz.acor.org/gestalt!/gerhards/archive.html ).