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