Flat feet is when the arch of your foot decreases and comes in contact with the ground. So when you make a footprint, you see your entire outline. No arch is visible.
Flat feet can be painful. In order to successfully manage flat feet, the most important thing is adequate shoe gear. The shoe gear should have a stiff sole, preferably a tied shoe, out of a more dense, durable material.
I also like over-the-counter inner soles. Depending on your weight, some of these over-the-counter inner soles need to be a little bit more dense, otherwise you'll flatten them out.
In my office for the treatment of flat feet we typically fabricate custom orthotics. A mold of your foot is taken, and we hold your foot in a neutral position, almost where you should be. From that mold we fabricate a piece of plastic that will fit easily inside your shoes and hold your arch up.
For more severe cases of flat feet braces are necessary in order to maintain the alignment of the foot. In severe cases or in a traumatic case of flat foot surgery would be indicated.