When you create a new article, you choose if this article belongs under another article.
For instance, if you create an article My first chapter and don't choose any page/chapter from the dropdown menu, this first chapter will get the text 1. My first chapter.
If you continue to do this - not choosing a page in the dropdown, next chapter will get the name 2. My second chapter.
So, what if you want to re-arrange a chapter? You just change the sort-number of one article and it will automatically adjust the chapter text.