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