"Aftermarket" is a term used to describe accessories that are not made by the factory. So for example, for your Suzuki Sidekick, if you buy accessories for your car that are not actually made by Suzuki, then those are termed "aftermarket". Many people use the term "Sidekick aftermarket accessories" specifically when referring to custom accessories for the Sidekick, like performance accessories or custom appearance accessories. But even if you just replace your Sidekick brake pads with OEM-grade factory-style brake pads that are not made by Suzuki, those too are aftermarket Sidekick accessories. Over the course of a vehicle's life, it almost certainly will have aftermarket accessories installed in it at some point, even if the vehicle is never actually "modded out".