Sorry for the lack of detail. When a new user creates an account, they are presented with an expansion of the Joomla profile questions. Mousing over the field name (see attached screenshot with red arrow) (examples: Nickname, Description, Signature, etc), there are tooltips / popups (highlighted in green on screenshot) with explanations of the content that should be entered.
You caught me in the middle of styling the tooltip and form, so the image is a little spare.