Value refers to lightness or darkness of the color.
In addition to the pure hue, there is a light, mid-range and dark value for each of the colors around the wheel. The light value known as a tint is made by mixing the pure hue with white. Mixing the hue with gray creates the middle value, which is called a tone. Mixing the pure hue with black produces the shade or darker values. Adding white, black or gray to a color does not change the hue, only the lightness or darkness of the hue.
When starting with a pure hue you produce:
Tint: light color, by adding white
Tone: medium value, by adding gray
Shade: dark value, by adding black
The gray scale is a tool used for determining color value. By placing the gray scale next to a color you can quickly determine its value (lightness/darkness). It is recommended that a designers use a gray scale with 10 or 12-steps.
Not all of the hues around the outside of the wheel have the same value. Yellow has the lightest value because it is closest to white and purple the darkest value because it is closest to black. That is also why yellow is placed at the top of the color wheel and purple is at the bottom. The remaining ten hues have values in between those two.
Intensely colored shapes appear larger than duller ones of the same size. Light-valued shapes seem to advance and expand while dark-valued ones seem to recede and contract.