The "Subpage Default" is your default sub page and displays the page content with a right sidebar. The "Subpage Fullwidth" displays the page content at full width of the page container. The "Index.php" is your home page template and functions the same as the "Subpage Fullwidth". The "Style Guide" page has been setup to display your site default styles.
The page will automatically display the page title at the top of the content area. The checkbox "Hide Page Title" removes the title from the page.
*NOTE - The index.php is only used for the home page.