Your server is configured to show files and directories, which may expose sensitive data such as login cookies to attackers in the cache directories. That has been fixed by adding a file named index.html to each directory. If you use simple caching, consider moving the location of the cache directory on the Advanced Settings page.
If you just installed WP Super Cache for the first time, you can dismiss this message. Otherwise, you should probably refresh the login cookies of all logged in WordPress users here by clicking the logout link below.
The logout link will log out all WordPress users on this site except you. Your authentication cookie will be updated, but you will not be logged out.