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