Tone is how dark the darks are and how light the lights are. Higher tonal range is often used in paintings to portray bright sunlight Like in this painting:
The paper and his legs are very bright, but the shadows beneath the bench are very dark.
This gives the impression of bright sunlight.
A narrower tonal range can be used to give a more atmospheric misty or hazy effect.
In this painting I have kept the shadows fairly light so they are closer in tonal range to the highlights, the two figures barely stand out and become a discovery within the painting.
The other most useful and powerful way of using tonal range is by creating depth within a painting such as in this one:
The trees and the bank in the foreground have a high tonal range whereas the farm and tres in the distance have a narrown tonal range which imparts a hazy effect and creates depth for the eye to explore, the foreground acting as a way of funneling the gaze. When this is combined with colour intensity as well it can be very powerful and give a wow factor to the image.
In the next section on composition I will talk about shapes.