It means that if your final variable is a reference type (i.e. not a primitive like int), then it’s only the reference that cannot be changed. It cannot be made to refer to a different object, but the fields of the object it refers to can still be changed, if the class allows it. For example:
final StringBuffer s = new StringBuffer();
The content of the StringBuffer can still be changed arbitrarily:
s.append("something");
But you cannot say:
s = null;
or
s = anotherBuffer;
On the other hand:
final String s = "";
Strings are immutable – there simply isn’t any method that would enable you to change a String (unless you use Reflection – and go to hell).