A flatter (or a colorist, plenty of colorists flat and
color) is a person doing the dirty boring work that comes before the coloring. Just flat areas of color where they need to be. Before you add in detail. You know, like light and shadow, gradients, effects and what have you. Some people prefer their stuff to actually look flat. Mike Mignola's Hellboy for instance is mostly and sometimes completely flat areas of color. But most people don't do that, for various reasons. For Mike's work it works, for most people's work it just seems lazy. This stage takes maybe 30 minutes to an hour at most for most pages, so you're not saving that much time. It is boring work, however. And some people actually don't have the time to waste on that so they hire a flatter from India or whatever real cheap.