Reaxe is a reactive UI framework like React, but for Haxe and OpenFL. It does not provide UI controls; it just makes it easier to keep presentation in-sync with state.
Haxe is the perfect language for cross platform development, and, while it's commonly used to build games, there aren't many libraries for using it to build user interfaces. Now that there are some OpenFL-based UI libraries like Feathers UI, it's time for a reactive UI framework to make working with OpenFL just as convenient as using Javascript and React.
The same way it's done in Javascript, just with less bloat. Since OpenFL is inherently component-based, there's no need for a 'virtual DOM', so Reaxe is just a simple step up from normal OpenFL programming. You can use Reaxe components just like you would normal OpenFL components, and you can use normal OpenFL components inside Reaxe components.
Want to learn how to use Reaxe to make OpenFL-based UI in a really convenient way? Check out the Getting Started Guide.