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