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