Twilio` is a cloud communication platform that allows developers to build and deploy voice and messaging applications. In this tutorial, we'll use Twilio's SMS feature to add phone verification to a Rails 4 app. The process involves creating a `PhoneNumber` model to store user phone numbers, generating a PIN, sending it via SMS, and verifying the entered PIN. We'll use AJAX to complete the process on a single page, making it seamless for users. The tutorial covers the creation of a controller, view, and JavaScript files to implement the phone verification functionality.