Password Protect Your WordPress without User Registration

You can password protect your whole site with a single password and also restrict users groups to access the site. The Password Protected WordPress plugin does the job for you.

Download Plugin Password Protect WordPress: https://wordpress.org/plugins/password-protected/screenshots/

Also look at this plugin from WPMU for password protecting components in a single page or post of WordPress:
https://premium.wpmudev.org/project/password-protect-selected-content/?project_version=b&utm_expid=3606929-3.h81fJH5hSeWKRLgbJFtxGw.1&utm_referrer=https%3A%2F%2Fwww.google.co.uk%2F