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