Shifter is a Static Site Generator and Serverless hosting service for WordPress.
This works by running WordPress in a temp environment for you to make edits and updates. When you are ready to deploy your changes Shifter creates a static version of your site and deploys that.
Only people with access to the Shifter Dashboard can see that PHP version of the WordPress site while site visitors see the static HTML version.
This allows your WordPress install to remain secure and unreachable by anyone who shouldn't have access and gives your static site version incredible speed and scalability.