The "Subpage Default" is the default layout for your subpage and displays the page content at full width of the page container.  The "Subpage Sidebar" template displays the page content with a sidebar on the right. The "Style Guide" page has been setup to display all the default styles available for your site.

The page will automatically display the page title at the top of the content area. You can use the "Hide Page Title" check box to remove the title from the page. The "Header Tagline" field allows you to add text under the page title.

Note - the Index template is only used for the home page.