Astro is an open-source JavaScript library for building cross-platform mobile applications. It is a lightweight library that makes it easy to create web-based mobile apps with HTML, CSS, and JavaScript. Astro provides a set of APIs that allows developers to build apps with native look and feel. It also supports features like push notifications, geolocation, and more. It is designed to be modular and extensible, so developers can easily customize their apps with plugins and extensions.