When you go for a ride, you probably pedal around with the pedals on the arch of your foot (middle part of your foot).
However, the best way to ride is on the balls of your feet, especially when you are standing and pedaling. When you ride on the balls of your feet you get a full range of natural body suspension. When you ride around on the arch of your foot (the middle part of your foot) or on the heel you will notice that your ankle is stiff and does not have much range of motion. Riding on the arch of your foot (the mid part of your foot) or the heel you won't be able to pedal hard when you need to. When you ride on the ball of your feet you will get a full range of ankle motion. Having proper foot placement helps stabilize the foot and reduces the risk of injury to your Achilles tendon, your calf muscles and your knee joint. It also increases the efficiency of your pedal stroke, so you produce more power using the same amount of energy on each stroke. You also want to ride with your toes pointing forward not out off to the side when you point your toes straight and pedal you will have full contact with the pedals versus when your toes are pointing out.