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