In GitBook, a space is the documentation site for one of your project. Each space then contains as many articles as you want (pages). They will all appear in the Table of Contents (ToC) on the left.
Pages can be nested and grouped. But before that you should know about the three different types of entry that you can put in the Table of Contents:
Pages: This is a basic article. It has a title and an optional description, and you can then put any content inside it.
External links: These entries are just links. You can use them to link to external websites.
Groups: These do not have content per se, but they serve to group pages together under a title. They don't have content themselves.
You can nest pages under each others by drag and dropping a page under another. The children of a page will be hidden and collapsed by default. There is no theoretical limit to page nesting. But we advise to avoid going deeper than 3 levels of nesting. Note that the homepage of your documentation (the first page) cannot have children.
Groups can only live at the top level of the Table of Contents. You cannot nest groups inside groups. Note that you cannot put your documentation homepage under a group.