Film grain
Fine random speckle laid over a picture, like the grain of photographic film.
See it in FX engine. Under Post-processing, raise Film grain.
In plain words
Old film was coated with tiny crystals that caught the light. In the picture they showed up as a fine, random speckle. Digital pictures are clean, and some look too smooth because of it. Adding a little grain makes them feel more like film. Grain also hides banding in smooth gradients.