The "Add Document Title" button allows you to set a title for your webpage. This will be displayed in the title bar of the visitor's browser whenever your page is viewed.
If a title already exists within the document you are currently working on, clicking the "Add Document Title" button will allow you to edit this title, otherwise a new title will be added. Only one title is required per page.
It is important to add titles to each of your webpages, as these titles will be used by Search Engines when indexing your site. If your site then features in the Search Engine's results for a particular search, visitors will be able to see at a glance the title of your page before deciding whether to visit you or not.
As such, it is best to keep titles concise and relevant to the overall content of the page, and NEVER use meaningless titles such as "Untitled Document", as some HTML editors default to - Google it and see just how many wegpages are out there with the title "Untitled Document"! Titles shouldn't, however, be a long description of the page's content - for this, see Other Document Information (META Tags). Titles should be short enough to fit completely within a browser's title bar without being truncated.