The optical center is the point on the lens where a light ray can pass through with no refraction. The PD and OC numbers on the prescription tell the optician where to place the optical center of the lenses within the frame so that the optical center is properly aligned with the client’s pupils. The PD number tells her how to align the lens horizontally, and the OC number (much more rarely used) tells her how to align the lens vertically. The exact procedure is rather technical. See, for example, this YouTube video on Vertical and horizontal decentration.