Noise
Random-looking variation that makes surfaces, motion and texture feel natural.
See it in Noise & texture. Pick the Clouds preset, then change Scale and Seed.
In plain words
Perfectly smooth colour looks fake, because nature is never that even. Noise adds variation: small ups and downs in brightness, position or colour. Some noise is rough, like TV static, where each value is picked on its own. Other noise is smooth, so neighbouring values stay close, like clouds or rolling hills. Tools stack and bend noise to make marble, wood, fire and water.