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.
Am I a single PD or dual 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.