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