Gloss differential occurs with pigment inks on glossy photo paper, satin /luster photo paper, or semigloss papers. In highlights or areas of white in a photo, the printer may leave these areas nearly void of any ink. When more dense coverage surrounds these areas, a patch that has much higher reflectivity (and takes on a shimmering look) can be seen on the paper if held at certain angles.
Generally, gloss differential is not a big deal as you have to hold the print at a fairly steep angle to see the effect. In a few cases though, gloss differential can be detected while looking directly at the print.
Newer pigment ink printers are much less likely to show gloss differential than older generation machines.