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