Want to build a beautiful website that has all the content stylistically presented? If that's what you want, there is no way around a thorough understanding of the structure of HTML elements and how you can manipulate their styling using CSS. Yet a good understanding of these elements and their styling requires a demonstrable mastery of the CSS box model.
Come to think of a case where you have so many boxes of different sizes, and you would want to arrange them to look beautiful. You're allowed to change their sizes, rearrange their content, give them new colors, and place them in positions you deem fit. Well, that's more of how the CSS box model works, and, therefore, it must form part of your learning curve if you're to excel in your CSS development.
If that speaks to you, this is your chapter - The CSS Box Model.
Tighten your seatbelt, and let's get going.