G for Grain
Grain, is the old age, film days term for what we know as Noise now. Since most of us, wont see grains in photos anymore, lets talk about the Noise. When we increase the sensitivity of the sensor, we make it responsive to stray signals too. Being increased over a certain limit, based on the cameras, the extra signals start to show and reduce the image quality. Often noise is evident in low light photos, and/or when highly tweaked. HDR gets around it often, but if it doesnt ruin the photo, leave it be.