This tutorial explains how to send SMS reminders from a PHP Symfony application using Twilio's SMS service. The process involves setting up a database, creating entity models for users and appointments, seeding the database with sample data, installing and configuring Twilio's SDK, creating a command to execute the sending of SMS messages, and scheduling the command to run at a specific time daily. The tutorial assumes familiarity with Symfony and provides instructions on how to install dependencies, configure the application, and test the SMS feature.