The "Subpage Default" is your default sub page.  The "Style Guide" page has been setup to display your site default styles.

The checkbox "Hide Page Title" removes the title from the page and the optional Header Sub Heading field.

• The index.php is your home template and assigned automatically to your home page.  This templates operates the same as your default template.
• The page field "Intro" is a wysiwyg section that displays above the content with a white background and the narrow full width container.
• The page content displays in the narrow full width container.  Most modules/layout templates use the content area, but display at the normal full width container.