This is a technique to give form to bodies or backgrounds.
It needs to be sparsely and carefully used. Practice.
A wonderful example is this drawing by E H Shepard from “Winnie the Pooh.”
To learn to draw shapes with hatch, create some ovals, then hatch and crosshatch as shown. Practice this, it is more difficult than it seems to get the hatching ‘relaxed’.
This helps the ‘drama’ of a drawing. It can make a picture stronger, alive or even more somber if needed.
The simplest form is to thicken the line where appropriate to give the impression of shadow.
The position of the shadow depends on a notional ‘light-source’ eg see picture below.
When you feel confident draw some characters in movement with shading.