Ionic is an open source mobile app development framework that allows developers to build high-quality apps for multiple platforms using shared code. It offers benefits like easy access to third party code and plugins, additional platform support, and fast development. Ionic uses AngularJS which provides reusable code that can be used across platforms. It also uses Cordova to host plugins for features like notifications, camera access, and more. Ionic provides a cost-effective solution for cross-platform mobile app development.