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. This means that developers can write their code once and deploy it across multiple platforms, saving time and effort. One of the key features of Flutter is its use of a rich set of pre-designed widgets, which help create visually appealing user interfaces. Additionally, Flutter uses the Dart programming language, which is easy to learn and offers strong performance, making it a popular choice for both new and experienced developers.