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