"Aftermarket" is a term used to describe parts that are not made by the factory. So for example, for your Geo Tracker, if you buy parts for your car that are not actually made by Geo, then those are termed "aftermarket". Many people use the term "Tracker aftermarket parts" specifically when referring to custom parts for the Tracker, like performance parts or custom appearance parts. 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 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".