Expo.dev is a platform that provides developers with tools and services for building React Native applications. By offering a set of libraries and workflows, Expo simplifies the development process for creating apps that work seamlessly across both iOS and Android devices. It allows developers to write in JavaScript or TypeScript, using APIs and services specifically designed to enhance mobile app functionality. The platform supports a managed workflow that abstracts many complexities associated with native app development, making it accessible for developers of varying experience levels. Additionally, Expo provides features such as over-the-air updates, push notifications, and device testing, significantly accelerating the deployment lifecycle.