An Expo server is a cloud-based platform used for building, deploying, and managing mobile applications. It provides developers with tools to create cross-platform apps for both iOS and Android using a single codebase. The Expo server provides a comprehensive development environment, including features like live reloading, testing, and debugging. Additionally, it offers features like push notifications, analytics, and over-the-air updates that make it simple for developers to maintain their apps. With Expo, developers can create high-quality mobile apps without having to configure, manage, and maintain complex infrastructure.