Depicting abstract ideas or realistic images in drawings and paintings often employs a system of values. In pen-and-ink drawings, value (or tone) can help evoke the illusion of light (bright or dark), the illusion of color, the illusion of depth (in focus or out of focus), and texture (surface details).
Shown here is one of many of my sketchbook pages where I experiment with ways to show values in an ink drawing. The simplest system is simply black and white (light and dark). The next simplest is a 3-step method with an intermediate middle gray (shown here). In a more complex work, there can be gradual blending of marks to approximate a wide range of tones.
Your assignment: Try a 2-step example, then a 3, a 4, a 5 a 15. Have fun.