G is for Grain
Grain is used in photography as a measure of the size of the photosensitive particles (generally silver) on film or photographic paper. With modern digital photography, one might compare it to the resolution of the image in pixels per inch.
Film with large grains will appear to be at lower resolution and less focused than those with smaller, finer grains.
The sensitivity of film to light is related to grain size. Films with low sensitivity to light work best in bright situations, like outside on a sunny day. These films have very fine grain structures and yield a sharp image. But, if one wishes to take photographs in low-light conditions, say around a campfire at night, a more sensitive film is needed. These films tend to have larger grain sizes resulting in a less-sharp image.
Grain can also be used for photographic effects.