Flutter is a free and open-source mobile application development framework created by Google. It allows developers to build high-quality, natively compiled applications for mobile, web, and desktop from a single codebase. Flutter uses the Dart programming language, which is also developed by Google, and provides a rich set of pre-built widgets and tools for building beautiful and responsive user interfaces. Flutter is also known for its fast development cycle, as it allows developers to easily build, test, and deploy apps to multiple platforms. It also has a growing community of developers and resources available to help developers learn and use the framework effectively.