Twilio Sync is a cloud-stored shared state technology that enables real-time collaboration and connection in apps. This article demonstrates how to use Twilio Sync with Express and React to show who's online in an app. The setup involves creating a backend using Express, generating an Access Token, and setting up a frontend using React. The React app connects to the Express server, which grants access tokens to clients. The app uses these tokens to create a shared state of online users, allowing real-time updates and connections.