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