It's a term that is used to describe the flaking of chrome off of a metallic surface over time. This flaking often resembles little "pits" not unlike pockmarks in the plating.
Ludwig snare drums made of Ludalloy (aluminum alloy) are notorious for their vulnerability to this condition since you generally cannot chrome-plate aluminum with lasting success without priming the surface with a plating of another metal. Even so, it may not takeāas we all have seen.
Here is an example of "pitting" on a Plymouth fender.