Flutter is an open-source UI software development kit created by Google. It allows developers to build natively compiled applications for mobile, web, and desktop from a single codebase. With its rich set of pre-designed widgets, Flutter enables the creation of visually appealing and responsive user interfaces. One of the key features of Flutter is its use of the Dart programming language, which offers a modern and efficient way to write code. The framework supports hot reload, allowing developers to see changes in real-time without restarting the application, thus speeding up the development process.