On certain pages (including this page!), a subtitle will appear under the title.

The subtitle is the text "< Manual:Skinning" that appears above the title.

To modify the subtitle, extensions can use hooks to call the OutputPage::addSubtitle method.

$wgHooks['BeforePageDisplay'][] = function ( $out, $skin ) {
	$out->addSubtitle( 'HELLO I AM A SUBTITLE' );
  };

For modifying the subtitle that appears on sub pages, such as this page, you can use Manual:Hooks/SkinSubPageSubtitle.