Your pupillary distance (PD) is the measurement of the distance between your pupils.
This is needed for the manufacturer of the eyeglasses to know where to place the optical center on each lens, so you can see well with the glasses.
What is a single PD or dual PD?
These are just two different ways to measure your PD.
A single PD it is the measurement, in millimeters, from the center of one pupil to the center of the other.
A dual PD is the measurement of the center of each pupil to the center of the bridge of your nose.
Watch this video on how to measure your own PD: