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