HTTP authentication is generally used to restrict access to site, it is generally used if site is intended for specific user groups or to restrict access to development server.

Drupal 8 provides contributed module named shield to restrict access. You can enable HTTP authetication using the following steps

  1. Download shield module from and place it in modules directory
  2. Login to your site as Admin
  3. From module list "admin/modules" enable "PHP Authentication shield"
  4. Goto "Admin > Configuration > System > Shield"(admin/config/system/shield)
  5. Add desired user name and password and save the configuration

Note: As of now Drupal 8 basic_auth module and shield module cannot co-exist together

You can find more Drupal 8 details here