Pupillary Distance (PD) is the distance between your pupil centers. This measurement is important when ordering glasses to make sure you are looking through the optical centers of your lenses. If you have glasses made by an optician they may take additional measurements, but those require the frame to be on your face, and standard values are used by online glasses suppliers for those measurements. This is fine for most people, but may be a source of discomfort for some.
Preferably do not use a single number for PD, use the separate distances for each eye measured to centre of the bridge of nose: many people are not perfectly symmetric"
Because your eyes converge when looking at nearby objects, your PD is going to be smaller when doing near work than for distance vision. If you are buying differentials or reading glasses your near PD should therefore be used.
Pupillary Distance and Frames
It also helps if you figure out what size frames fit your head. One trick I read online is that the PD of the frames themselves in mm units is the sum of the first two numbers on the frame. So if you have frames that are 53-15-140, the PD of the frame is 53 + 15 = 68mm. In general, you’ll do better with frames that are close to your actual PD: your eyes will be centered in the frames, and the lenses will be centered in the frame too.
Measuring your PD
See Jake's blog post Headaches & Fatigue: Measure Your Pupillary Distance (PD).