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