The Send Messages Extension on Firebase, developed by Twilio Lab, is a pre-packaged solution that automates the process of sending messages (SMS or WhatsApp) based on information from documents in a specified Cloud Firestore collection. To use this extension, developers need to install it and provide configuration parameters such as cloud functions location, Twilio account SID, Twilio auth token, and message documents collection. The extension allows users to add documents to the specified collection with specific formats including phone numbers, bodies, and media URLs, which are then used to send messages. Once a message is sent, the document in Firestore is updated with delivery status information. To test the extension, developers can manually add documents to the collection or use the Firebase Admin SDK to automate the process.