A Sub-topic is what we call a page (Tiddler) that has been tagged with the title of another tiddler; being tagged by 'A' denotes 'B' as standing underneath Topic A in the hierarchy

That other tiddler (if you are Editing it is the title you type in the "tags" box) is known as the Topic, and is the 'headline' over any other tiddlers that it tags.

See the Show references and info panel to reveal lists of topics and sub topics.