Alan De Marche
Value refers to the gradations of light to dark. We generally focus on a "value scale" or "gray scale" of 9 to 10 values; 1 being white and 10 being black. Furthermore, values 1 - 5 are referred to as high key values whereas 6 - 10 values are low keys. The Italian word for light and dark is "chiaroscuro."

Value plays a number of roles within any given image. Values may 1. provide a sense of light as in the time of day, 2. create volume when gradations - transitions between light and dark- are applied.  And 3. values can be used "subjectively" for dramatic effect or psychological impact. For instance, lighter (high key) values are more cheerful and uplifting whereas darker values (low key) can be more ominous or mysterious. When creating depth, we generally see dark  values as receding and light values advancing.