"Aftermarket" is a term used to describe accessories that are not made by the factory. So for example, for your Mercury Milan, if you buy accessories for your car that are not actually made by Mercury, then those are termed "aftermarket". Many people use the term "Milan aftermarket accessories" specifically when referring to custom accessories for the Milan, like performance accessories or custom appearance accessories. But even if you just replace your Milan brake pads with OEM-grade factory-style brake pads that are not made by Mercury, those too are aftermarket Milan accessories. Over the course of a vehicle's life, it almost certainly will have aftermarket accessories installed in it at some point, even if the vehicle is never actually "modded out".