The Table of Contents (TOC) in most projects is organized in chapters with sub-chapters. You can change the structure by moving topics around and by changing their levels. Changing the topic level is referred to as "promoting" or "demoting" a topic – promoting a topic moves it to a higher level in the TOC, demoting it moves it to a lower level.
Topics with sub-topics are automatically displayed as "chapters", shown with a book icon in the TOC.
You can also organize topic files in the Topic Files section in folders and sub-folders, but this is for convenience only. It does not have any effect on the structure of your project.