A template is a webpage design and layout, which can be used for many pages of your website. Most templates have a space which you'll fill with content.
All sites have a "Home" template - it doesn't have a content space and is only used for your website's homepage. You can manage elements of this page using Template Builder.
One of the templates on your site is set as the default, which means it is automatically selected when you create a new page. You can change the template for each individual page though, by going to the Advanced section.
Templates are automatically set up when you select or change your website's theme in Template Builder. You can't modify the HTML of a template directly.