If you look at an adult foot from the inside, you'll usually notice an upward curve in the middle. This is called an arch. Tendons -- tight bands that attach at the heel and foot bones -- form the arch. Several tendons in your foot and lower leg work together to form the arches in your foot.
When the tendons all pull the proper amount, then your foot forms a moderate, normal arch. When tendons do not pull together properly, there is little or no arch. This is called flat foot or fallen arch.