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