Flutter is an open-source UI framework created by Google. It’s used to develop applications for Android, iOS, Linux, Mac, Windows, and the web from a single codebase. Flutter provides high-quality native interfaces on any platform in record time, allowing developers to create beautiful, natively compiled applications.

Here are some key features of Flutter:

1. Fast Development: With Flutter’s hot-reload, you can experiment, build UIs, add features, and fix bugs faster.

2. Expressive and Flexible UI: Flutter comes with rich, customizable widgets to build native interfaces.

3. Native Performance: Although it’s a cross-platform tool, it allows developers to maintain superior native performance.

4. Single Codebase: Write one codebase for your app to create a multi-platform application.

5. Dart Language: It uses the Dart language which has strong support for creating asynchronous tasks.

6. Backed by Google: Being backed by Google, Flutter has a large community of developers and robust documentation.

It’s often used by developers who want to save time and resources by developing efficient cross-platform apps that feel native on each platform.