What does "optional operation" mean in Javadoc of for example Set#add(E)?

When in the java documentation for Set it says in the specification of a method Optional Operation e.g. (emphasis by me)

add(E e)
Adds the specified element to this set if it is not already present (optional operation).

What does the optional mean here?

That if I use a JVM other than SUN/Oracle, this operation may not be provided by that implementation of Java?