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