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