Filed under: Star Trek.
His answer was, “When an act is morally praiseworthy but not morally obligatory.”
And, in fact, there is a term in ethics for such acts: Supererogation (super-er-o-gay-shun), from the Latin meaning to pay out over and above. So the question must have been, “What is supererogation?” or words to that effect.
Now you know. :)