It is the layers of skin beneath the outer layer that produce melanin, which affords protection from harmful ultraviolet rays. The darker the skin, the more melanin, hence more protection from the sun. The more you are exposed to the sun, the darker you can get. So the darkest people have the most protection from the sun, the fairest the least. Those living around the equator are usually darker than those living further north.
Some people produce no melanin.
I encountered this the other day. To the best of my knowledge, they are referring to whether the glasses are prescription or not. I just spoke with a friend who is an optometrist, and she says she hears it constantly.