I'm not the author, but if you want to learn how something specific about Bootstrap works, i'd recommend inspecting a page with the browser's dev tools. You can immediately see which styles apply to each element, its paddings and margins, and play with the classes' style values and see how they affect its positioning: http://imgur.com/ZzkF0Jb.png

This approach has helped me a lot in understanding how many of Bootstrap components work, and why they are structured the way they are :)