Flutter is a free and open-source mobile application development framework created by Google. It allows developers to build high-performance, visually attractive, and natively compiled mobile apps for both Android and iOS platforms from a single codebase. Flutter uses the Dart programming language, which is also developed by Google, to build mobile applications.

Flutter's main advantage is its ability to build apps quickly and easily. The framework provides a rich set of pre-built widgets and tools that simplify app development and help developers create beautiful and responsive UIs. Flutter also supports hot reloading, which means that developers can see the changes they make to the code in real-time, making the development process faster and more efficient.

Flutter also offers excellent performance and speed. The framework's widgets are designed to run at 60 frames per second, providing a smooth and seamless user experience. Flutter also uses a unique feature called "Ahead of Time" (AOT) compilation, which compiles the Dart code into native machine code, resulting in fast and efficient apps.

In summary, Flutter is a powerful mobile application development framework that offers high performance, fast development, and visually attractive UIs. It is an excellent choice for developers looking to create beautiful and responsive mobile apps for both Android and iOS platforms.