The "Subpage Default" is your default sub page with a sidebar and the "Subpage Fullwidth" displays the content area the full width of the container. 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. The page description field is optional.
*NOTE - The index.php is your home template and assigned automatically to your home page. This templates operates the same as your "Subpage Fullwidth".