Flutter is an open-source UI software development kit created by Google. It is used for building high-performance, high-fidelity, apps for Android, iOS, web, desktop, and embedded devices from a single codebase. It allows developers to write once, run anywhere and helps them to build beautiful and fast applications. It uses the Dart programming language, and its reactive programming paradigm helps in building flexible and responsive user interfaces. It is a popular choice for developing cross-platform mobile apps with a native feel.