Then I guess you didn't click on the documentation link at the top of the roboVM page.

Why RoboVM? We love mobile. We love the Java ecosystem. If you feel like we do, you will love RoboVM:

Create truly native mobile apps for iOS and Android. Use libraries, tools and languages from the vast Java ecosystem. Share code between platforms.

It seems to be an alternative to xamarin focused on Java or at least the JVM.