The nap is the fiber pile on a roller cover that picks up paint and transfers it to the wall. Short naps suit smooth surfaces, while longer naps reach into textured walls like stucco. Matching the nap to the surface is a small choice that shows up in the final coat.
The wrong nap leaves the finish either stippled or thin. Too long a nap on smooth drywall adds texture you didn’t ask for, and too short a nap on stucco misses the deep areas entirely. It’s a basic supply item that quietly decides how the wall ends up looking, so being particular about length is crucial during the painting process.