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