Assigning a gray value to an area based on how bright or dark it is. Do not confuse this with shading. Shading deals more with light and shadow. Think of value as color. Brown (tree trunks) would be a darker tone than tan (grass).
Notice in this picture, everything has a tone based on its brightness. The darker green grass is a mid-gray and the light tan of the cliffs is more of a white tone, but both still have highlights and shadows from the sun.