What do dark tones, shadows and bright tones mean respectively?

Highlight is the bright part of the picture, and dark tone is the dark part. Because of the different illumination, an object will have bright parts and dark parts, so it looks real. So it's actually light and dark. Relatively speaking, white is brighter (highlight), black is darker (dark tone), and gray is a shadow tone, which is just in between. For example, brighter, darker, brighter than black. This is the shadow.