Flutter is an open-source UI software development kit (SDK) introduced by Google in 2017. It is widely used for building cross-platform applications with a single codebase, allowing developers to create apps for Android, iOS, web, and desktop simultaneously. Flutter is popular because it offers features like hot reload, a rich widget library, and native performance. Its ability to create consistent user experiences across multiple platforms while reducing development time and cost makes it a preferred choice for developers and organizations alike.