Values are lights and darks that give form and depth to a subject. Observing the wide range of values that make up your subject will give you a better understanding of how to complete your drawing.
The way you use contrast affects the appearance of the subject. More contrast makes for a broader range of values: The features are sharper and more distinct, and the drawing is more dramatic. With less contrast, the range of values is narrower. The overall feeling is softer and with less depth.
Values can appear different depending on their surroundings. A silhouette of a person in a gray value may seem dark when placed on a white background. That same silhouette seems lighter when placed against a darker background.
A value scale is used by holding it against the subject and comparing the values with those of the drawing.