What that means is that the tookkit makes it easy to write secure applications independent of the details of the underlying security algorithms. You can write a single program, and it can encrypt using single code. If someone is paranoid about security, they may want to use a Java security engine that they trust. We allow the designer to choose what happens under the covers. On the other hand, you don't want to stick to the sandbox paradigm, just because that code can't do any damage.
Sandboxes are for toys.
Our security system allows you to control where the executable content goes, and what it can do. For programmers, it's marvelous. When programming with the older languages, there was a whole class of bugs present. One possible future for Java is that it becomes the programming language for all matters of development, because it's a safer language. That is one part of Java that is secure.