Does just thinking about nap make you sleepy and you wonder what in the world it has to do with sewing?  Nap has to do with a fabric's grainline and texture.  If it has any kind of pile to the fabric, no matter how thick, it is considered to be a fabric with nap.  Examples of these are suede, velvet, velour, and corduroy.  You have to treat these fabrics differently than fabrics without nap, especially when it comes to cutting out your pieces.  You will learn that if you don't cut out your pieces along the same direction of the grainline, your pieces will look like two different colors when sewn together  It's a cool effect but not one that we want on accident. Learn how to sew it the correct way.