We're putting together more docs to help cover these concepts soon!
Single thread (which we call a moot) doesn't support threading. If you try to browse bigger threads here in HN or in Reddit you'll soon realize that you'll get lost. Especially when you come back to the site it's impossible to know what has changed. We think discussions should be chronological.
Later we're adding support for replying to replies (one level only) and a way to branch from the main topic.