This tutorial teaches developers how to create an appointment booking system using Twilio, Notion, and FastAPI. The system allows users to book appointments via WhatsApp messages and receive updates on the status of their appointments. The system consists of three main components: a backend API built with FastAPI, a database stored in Notion, and a WhatsApp messaging service provided by Twilio. To create an appointment, users send a message to a specific phone number containing the user's name and phone number, which are then used to create a new entry in the Notion database. The system also includes endpoints for retrieving appointment details and sending status updates. Throughout the tutorial, developers learn how to use various APIs, including Twilio's WhatsApp API, Notion's API, and FastAPI's built-in features. By following this tutorial, developers can build their own appointment booking systems using these technologies.