Flutter is a new mobile app development framework created by Google that allows building high-performance, native apps for iOS and Android from a single codebase. It uses Dart as its programming language. Flutter compiles code to native ARM machine code rather than interpreting it, allowing apps built with Flutter to perform similarly to those built natively with Swift or Java. Flutter provides widgets that are analogous to HTML/CSS elements, allowing developers to build custom user interfaces easily. Flutter also supports features like hot reload, which instantly pushes code and UI changes to the emulator or device.