"Aftermarket" is a term used to describe parts that are not made by the factory. So for example, for your Volkswagen Vanagon, if you buy parts for your car that are not actually made by Volkswagen, then those are termed "aftermarket". Many people use the term "Vanagon aftermarket parts" specifically when referring to custom parts for the Vanagon, like performance parts or custom appearance parts. 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 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".