What is Flutter? Flutter is a UI toolkit for building natively compiled applications for mobile, web, and desktop platforms with a single codebase. It represents the most recent effort towards what Java promised: a “Write Once, Run Anywhere” experience. Writing a single codebase that runs on many platforms offers a number of advantages. Traditionally, if an organization wanted to support their application on web, mobile, and desktop it would have to support 3 separate codebases all in different languages and using different technologies.