Jogger's nails: Very small semi-circular white spots on the toenails. The white spots on the nails reflect injury to the base (matrix) of the nail. The matrix is the part under the visible nail where the nail cells and the nail itself are produced.
The injury to the nail matrix responsible for the white spots is often gotten during exercising in poorly fitting shoes. Jogging in badly fitting shoes is notorious. It causes this condition so often it is known as jogger's nails."
Jogger's nails are no cause for concern. They eventually grow out. However, they are a useful sign, that shoes that fit right are in order.