Graininess refers to the amount of grain found in a photograph. Grain is a film photography term, and its digital counterpart is “noise,” though the two terms are sometimes used interchangeably in digital photography. Grain appears as tiny flecks throughout a film photograph. It can vary in its texture and amount depending on the type of film used, particularly its speed (ISO, formerly known as ASA,) and how it is processed.
Despite the fact that it does not technically apply to digital photographs, in Lightroom, there are “grain” sliders in the Effects panel for use in film emulation style processing.