what does this mean?

This is privacy score out of 21.

If your score box is green then you have scored 21/21 and all your information is hidden from websites and the public. Excellent.

If your score box is yellow then you have locked down most of your information. Good stuff.

If your score box is red then your are exposing lots of your private information and you should immediately review your Facebook privacy settings.

Note because Facebook does not let you hide all your information, you cannot get a score a perfect 21. With everthing locked down the best score that you are likely to achieve is 15/21 (unless you have not filled in profile information such as your gender,your likes or you don't have any friends; then you can score higher.) !