Flutter is an open-source UI software development kit (SDK) created by Google, used for building natively compiled applications for mobile, web, desktop, and embedded devices from a single codebase. Flutter allows developers to create visually appealing and highly performant apps using the Dart programming language, featuring a reactive framework and a rich set of pre-built widgets. With its "hot reload" feature, developers can see real-time changes during the development process, enhancing productivity. Flutter is popular for creating cross-platform applications that deliver a native-like user experience while maintaining consistency across platforms.