Contour lines:

source:

I have made a small gif work in Photosop to have a visual explanation of what condour lines are really about, in practice.

Are called the lining you usually see on architectural design and at topological mapping, but not just there,
contour lines are the lines that shape the forms you see and are translated in the liner format.
Here on steemit (everyone love him like I do) our fellow and talented steemer mr @nekromarinist is a master of that technique, shadowing his sketches with it and creating some wonders with just some simple lining work. I will display an old piece of his artwork, and I stronly suggest you to give him a look and follow him, he is very creative and inspiring. He always deliver.

credits @necromarinist

So what is the theory behind contour lines?

Well simply put you need to follow the shape of the object while creating your shadowing lines. For that reason you can practice out on any object you see by braking it down on lines then shading it.
I will include some basic shapes here for displaying purposes. with some fast contour lining shadows.

practice makes perfect.

best regards.
@skapaneas