The Outline tool is for organizing and structuring your content. Typically with word processor, you organize your content with file directory system. However, as your book evolves, it gets difficult to manage and rearrange files. The Outline tool allows you to come up with a blueprint of your book and write your content piecemeal. The following video demonstrates creating new sections.
Once you get familiar with the Outline tool and hierarchy options in the Section Properties panel, you will at some point wonder whether to keep one big section or break it into multiple sections. A good rule of thumb is to create a section for every major point— just as you do in a plain outline. Down the road it will become easier to have your idea broken down into multiple sections rather than all in one section.