The libGDX project is a cross-platform game development library written in Java, with some JNI code for performance hungry sections. It abstracts away the differences between writting desktop, Android and HTML5 games based on standards like OpenGL ES/WebGL. Applications can be prototyped and developed entirely on the desktop, then only 6 lines of code are needed to run your app on Android or HTML5.
Its a great framework and has the best performance of all other frameworks and engines out there, pretty much.
Its bad things are that it doesnt have HotSwap when using Gradle, it doesnt have Windows Mobile build (or at least i dont know about it) and some others, but in general its as powerful as any other engine/framework and most of the times better.