"Aftermarket" is a term used to describe accessories that are not made by the factory. So for example, for your Geo Tracker, if you buy accessories for your car that are not actually made by Geo, then those are termed "aftermarket". Many people use the term "Tracker aftermarket accessories" specifically when referring to custom accessories for the Tracker, like performance accessories or custom appearance accessories. But even if you just replace your Tracker brake pads with OEM-grade factory-style brake pads that are not made by Geo, those too are aftermarket Tracker accessories. Over the course of a vehicle's life, it almost certainly will have aftermarket accessories installed in it at some point, even if the vehicle is never actually "modded out".