Flutter is an open-source framework developed by Google for building natively compiled apps across mobile, desktop, and web platforms using a single codebase, offering flexibility and rapid development through features like hot reload. It provides a rich catalog of customizable widgets to create responsive user interfaces and is cost-effective, making it suitable for startups and MVP development. With strong support from Google, an active community, and continuous updates, Flutter's future in cross-platform mobile app development looks promising.