Shelving is the change (step) in height of the surface of the porcelain as you move across the sign. It is seen at the boundary between two colors, because the porcelain ("frit") is applied in layers, one color at a time. So, for instance, on a porcelain oil well lease sign, the black letters are applied on top of the white background, so the surface steps up as you move from the white area to the black letter.
As a general rule, the older porcelain signs have heavier shelving than newer ones. The height of the shelving decreased in the 40s and 50s, and by the 60s, most porcelain signs had very little noticeable shelving.
Painted tin signs, while the colors are also applied in layers, generally have very thin paint or ink, and display little or no shelving.
Beware, though, as there are fake porcelain signs with intentionally heavy shelving -- especially those from South America.