Pages that have a password set can only be viewed when the password is entered.
You can activate password protection by going to the Pages section and clicking the Protect Page on the page you want to protect. This password will protect this page and all the subpages.
Passwords are ignored in the dashboard's live site preview, since you're the only one viewing it. Adding or removing a password will also add or remove it from all child pages of the set page.
You can also decide whether to have your navigate be visible even on protected page.