In this guide, Twilio Add-ons are introduced as pre-integrated third-party services that enhance the responses from Twilio by including additional data from partners. The IBM Watson Speech to Text Add-on is used in an example to convert audio recordings into text using machine intelligence. To use an add-on, it must be installed on a Twilio account and configured with a callback URL. Once installed, the add-on's data can be accessed within an HTTP request made to the application. The guide also provides information on how to enable add-ons in the Twilio Console and how to use their data in a C# (ASP.NET MVC) web application.