PD stands for you pupillary distance. This is the distance between your two eyes when you are looking at something near (near PD) or in the distance (distance PD). It is circled on your prescription because your glasses need to be made up for close work and so the lenses will need to be glazed to so that the optical centres of the lens matches your near PD. This will give you the best vision as you will be looking through the correct part of the lens