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