The tutorial by Matthew Setter demonstrates how to enhance a PHP application using the Mezzio framework to respond to SMS messages with Twilio's API. Building on a previous guide that covered sending SMS, this tutorial focuses on creating a webhook that listens for incoming SMS requests from Twilio and sends an automated reply using TwiML, Twilio's XML-based markup language. It involves setting up a new HTTP POST endpoint, updating the TwilioService to generate SMS responses, and configuring a new request handler class named SMSReplyHandler to process incoming requests. The process includes updating the routing configuration and testing the functionality using ngrok to expose the local server to Twilio's webhook. The guide also provides debugging tips via Twilio's console to resolve any issues that may arise. Overall, the tutorial is a concise exploration of integrating Twilio's SMS capabilities with PHP's Mezzio framework, offering insights into both technologies.