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