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