For Specific PagesYou can make a page private or password protected, which will require any users visiting the page to enter a password you create. When editing any of your pages, you will see a lock icon that you can click on to enable password protection.
Site-wide PasswordIf you'd like to protect your entire site with a password, go to your Pages Tab and find the Site Password switch on the right-hand side.
Once you switch it on, just enter a password and click save. This will require your visitors to enter a password before they can see any part of your site.