"Aftermarket" is a term used to describe parts that are not made by the factory. So for example, for your Chrysler 300m, if you buy parts for your car that are not actually made by Chrysler, then those are termed "aftermarket". Many people use the term "300m aftermarket parts" specifically when referring to custom parts for the 300m, like performance parts or custom appearance parts. But even if you just replace your 300m brake pads with OEM-grade factory-style brake pads that are not made by Chrysler, those too are aftermarket 300m 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".