This is an exercise to help you develop stronger feet, particularly the arch of the foot. If you pronate this is an especially good exercise for you. Imagine dots at the big toe and pinkie toe joints where they meet the feet, and a dot at your heel. Now, imagine pulling the dots at the front of your foot towards the heel.
At first you might not see lots happening, but over time you will see the arch of the foot pull away from the floor.