"Aftermarket" is a term used to describe accessories that are not made by the factory. So for example, for your Ford Five Hundred, if you buy accessories for your car that are not actually made by Ford, then those are termed "aftermarket". Many people use the term "Five Hundred aftermarket accessories" specifically when referring to custom accessories for the Five Hundred, like performance accessories or custom appearance accessories. But even if you just replace your Five Hundred brake pads with OEM-grade factory-style brake pads that are not made by Ford, those too are aftermarket Five Hundred 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".