Do you ever doodle automatically on a piece of paper while you're phoning? That's the same principle as hatching: drawing parallel or spontaneous lines to create darker areas.
The closer together you draw the lines, the darker the overall shading will look. Conversely, the farther away you space your lines, the lighter the shading. Hatching allows you to shade your drawings.
To tone down hatched areas, from darkest to lightest, you can:
Freehand hatching: for a more luminous drawing.
Curved hatching: ideal for adding volume to the drawing of a round object.