Tone (or value) is the degree of lightness or darkness in a colour. When using pen and black ink, the palette ranges from white through the various grey tones to black.
When trying to understand tone, it is useful to start with one colour on white paper. In my short video, I create a very simple drawing of a tree in black on a white background.
The darkest tones are where the light reflects the least on the tree and the lightest tones are where the light reflects the most on the tree. In this case the darkest areas are around the back of the tree and the lightest areas run down the centre. The mid tones are the grey tones around the middle.
When using pen and ink, dark tones are created by placing the lines closer together and combining different layers. The lighter tones have less strokes with the pen.
Adding charcoal to the background gives more depth in certain areas.