The Java Embedding Plugin is a utility that allows other web browsers than Apple's Safari to use the most recent versions of Java on Mac OS X. When used together with an updated version of Mozilla's MRJ Plugin Carbon (included in this distribution), the Java Embedding Plugin's functionality is currently available to recent versions of Firefox, Seamonkey and Camino. But in principle any web browser could use one of the Java Embedding Plugin's two APIs to add support for Java 1.4.2, J2SE 5.0 and (where available) Java SE 6.
The current version (0.9.7.5) of the Java Embedding Plugin and the MRJ Plugin JEP requires Mac OS X 10.4.X or higher. Older versions (0.9.6.5 and earlier) required Mac OS X 10.2.8 or higher.
The Java Embedding Plugin has for the last few years been bundled with current Mac distributions of all the Mozilla.org browsers -- Firefox, Seamonkey and Camino. So if you're using one of these browsers, you don't need to install the Java Embedding Plugin. But you may wish to replace the bundled version of the Java Embedding Plugin with a more recent version.
For more information see the Java Embedding Plugin Readme
or go to the SourceForge project site.