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