What does "no-op" mean?
"No operation", i.e. do nothing.
It comes from the assembly language instruction "noop" on most processors which does nothing for one clock cycle and is used for example in delays.
This seems to be vaguely computer-science related. I think it means that "Add" is a non-operation. In more words, the "Add" operation does nothing to an object in the "Added" state. Does that answer your question?
Edit: Martin's answer is better.