"Aftermarket" is a term used to describe parts that are not made by the factory. So for example, for your Porsche Cayenne, if you buy parts for your car that are not actually made by Porsche, then those are termed "aftermarket". Many people use the term "Cayenne aftermarket parts" specifically when referring to custom parts for the Cayenne, like performance parts or custom appearance parts. 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 parts. Over the course of a vehicle's life, it almost certainly will have aftermarket parts installed in it at some point, even if the vehicle is never actually "modded out".