Page's privacy is a way for you to limit access to perticular pages, it can be very useful if you want to make some private discussions groups targeting only some of your users.
This is the default behaviour, a public page will be accessible for all your users, it can be followed if you have activated the Follows behaviour. You can also invite users in the page.
A private page will require your users to send a request in order to being accepted inside this page. All the content inside it, is invisible to a non accepted user. You can also invite users in the page
A secret page will never be indexed on your page listing. The only way for a user to be a follower of this page, is to be invited by the page owner. All the content inside it, is invisible to a non accepted user