Tone:

Tone referes to light and dark values used to highlight or define a piece of art. Adding tone to a piece of art can make the character, object or landscape stand out more and can be used to define certain areas of the image or even show light. Tone is particuly good at demonstrating light, for example; on a character its good to show where shading should be and define certain partsof the body. Tone is usualy demonstrated with a black pencil or pastel to capture the shading and define the light but you can also use a brown pastel or pencel too.

Aslo, Tone is very good for demonstrating contrast in an image, for example; on a piece of landscape art you would have the furthest away part in white and the closet part of the image in black because tone goes from pure black down to white. Bellow is my own demonstration of how the contrast of tone works starting from the lightest dark to pure black.