The text discusses how to extend a Mezzio-based application to send an SMS reply when an SMS is received by the application's Twilio phone number. The process involves adding a new method to the `TwilioService` class, creating a new handler class to receive and respond to requests from Twilio, registering a new route in the routing table, and testing the changes. The application uses Twilio's SMS API and PHP's Mezzio framework to send and receive SMS messages. The tutorial covers the steps to implement this functionality, including debugging tips for common issues that may arise.