A foot with a low arch is a foot with a structure that collapses each step taken. The bones that make up the medial longitudinal arch of the foot are supposed to stay in a semi-rigid arch structure while walking.
Flat Feet (also known as pes planus) is a condition where the foot lacks an arch on the inside. It is a very common condition as almost all children start out with little or no arch. Most kids eventually develop a normal arch; however, often people grow to maturity without the development of a normal arch.