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