The Template Manager creates and inserts templates - custom html files that contain content for inserting into articles, often text or html that you use frequantly like disclaimers, copyright notices or structured html content to build articles on like layouts, tables etc.
Templates can be created either by saving an articles html content to a file, or by creating the template (an html file) in an HTML editor. For more information on creating templates see Creating a Template.
The Template Manager is not a plugin to create or edit Joomla! Templates