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