It's just what base colors you use. Most people start with a set of red, yellow, blue, black, and white. (Replacing green with yellow, but they still call it RGB for whatever reason.) CYMK in paint is just that - cyan, yellow, magenta, black, and also a white. You use those colors to mix your primaries instead of starting with them. It's not really the same as how RGB works, because you can't make yellow with green in paint, but enough designers would come into my old store referring it that way, that I just learned to understand what they meant.
This was mostly older designers, mind you, and they phrased things oddly. Such as calling a roll of tracing paper "bum wad".