The foot is a very complex structure. It is comprised of many small bones and ligaments that interact with each other, unlike most other joints in which only two or three bones interact. This makes the foot very adaptable to different surfaces.
The foots main function is as a shock absorber, it takes up all the force of the body's weight landing on it. The foot supports up to four times your body weight when you run and it must bear up under at least 1,800 foot strikes for every mile you walk or run. Many overuse injuries of the lower limb are due to an abnormality in the way the foot hits the ground.
Your physiotherapist will explain your injury and what they can do to help you.