Now it seems that a person is not morally obligated to throw himself on the hand grenade, but if he does we would acknowledge that he has done something virtuous and good. He has sacrificed his life on behalf of others. He was not obligated to do so, such that if he did not do so, we would not find fault with him. But since he did do this, then we can applaud his actions. This is what heroism is.
Heroism is when somebody does something above and beyond the call of duty.
He does something that is not morally required of him, but we can't find fault with him if he didn't. When you call her acts heroic you are saying that what she did was not obligatory. People could have driven by as they did, and they carry no moral blame for doing so. In other words, when we say that what somebody does is heroic we are saying that the action itself they perform is not obligatory. This is a human being who needs our help and we are obliged to help her.