Rhino is an implementation of JavaScript in Java. Documentation can be found here.
The purpose of this fork is to provide a release version of the 1.7R5 branch in the Maven Central repository. Rhino has been languishing for a number of years now without a release.
The ejs.io project is focused on Enterprise Javascript running on the JVM, specifically around the RingoJS platform. Before we can push RingoJS into the central repository, we need its dependencies (namely Rhino 1.7R5) to exist in Central.
So that is why this fork exists.