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