You must have a Private SSL certificate correctly installed or enabling this option will render your site inaccessible.
Admin SSL forces wp-login.php and wp-admin/profile.php to be secured (these are the pages on which you can enter a password). When HTTPS is being used, the content and includes directories are also secured. Here you can add other URLs to be secured by Admin SSL.
If you want the Admin SSL buffer to ignore a particular page, enter its URL here. The page output will be completely unaffected by Admin SSL - i.e. all links will be left as 'http'. However, the page itself will still be forced to SSL if it is entered in the 'Additional URLs' box above.
This feature is particularly for xmlrpc.php, but may be used if other pages or plugins require it.