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