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