The object referred to by this property sits at the top of the java package name hierarchy. It is through this property that you can access the java objects, properties and methods via LiveConnect.
This short-cut reference corresponds to a directory hierarchy where Java class packages are stored. Thus the java.lang.String class lives in a file called java/lang/String.class, which on some systems may be stored inside a ZIP archive.
The main shortcoming in this whole mechanism is that the browser cannot tell whether a reference to an object is a request for a JavaPackage or a JavaClass. It will assume you mean a package by default and if you misspell a class name, it won't tell you it couldn't find it.
The objects and classes supported by this access to the underlying Java engine cover a very wide topic base. We have examined only the top level functionality to try and establish points of connection between the two environments. For a full and in-depth reference coverage of the Java language environment, consult the Wrox book "Java Programmer's Reference" by Grant Palmer.