Contour lines are drawn around the form of an object to clarify its volumetric shape. This is helpful to sketch in before painting when you're not sure about how something should be rendered, and it's a great exercise to practice working spacially while drawing! They're also helpful to use when adding details so you know the features you're drawing are sitting on the form correctly, conforming to the surface beneath it well.
This is part one, I'm planning on doing at least one more version of this: one to practice more rectangular forms, maybe one for more organic forms. Here's an example of contour lines in action:

I hope this helps! Feel free to always leave comments and questions below :)