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