Simply put, Roller Nap refers to how thick is the fabric material. The shorter the nap, the smoother the finish will be. However, the cover will hold less material requiring more "dips" into a roller tray to refill the cover.
Covers with less than a 3/8" nap are typically only used for enamels.
Most consumers prefer to use a 3/8" to 1/2" nap cover for walls and ceilings. These naps produce very little "orange peel" texture.
Most professional painters will use a 3/4"nap cover because it holds more paint allowing them to work the wall longer between reloading.
Rough surfaces will require 1" and 1-1/4" nap covers to fully apply paint to the surface. While these covers will leave more texture profile, it is not as noticeable on a rough surface. However, if these are used on smooth surfaces, you will notice this texture.