Good means helpful.That's the bulk of it. If I'm asked what it means to call something good, my response is, "It means that thing is helpful."
Of course there is a difference between 'good' and 'helpful.' People are much more likely to ask the obvious follow-up question, "Helpful for what?" than they are to ask "Good for what?" In some contexts, it's downright jarring for someone to ask the follow-up question about 'good,' e.g:
"Helping the suffering is good."I suggest this difference doesn't come from the meaning of 'good' itself. Instead, we associate 'good' more strongly with certain conventional answers, while we don't associate 'helpful' so strongly with any particular answers. 'Helpful' is the more open-ended form of 'good.'
"Good for what?"
"How can you ask that?!"
One of the major conventional answers to "Good for what?" is "reducing unnecessary suffering." This is why the above statement sounds so obvious. Spelled out, it would be:
"Helping the suffering is helpful [for reducing unnecessary suffering]."If some idiot or philosopher asks, "Good for what?" in such an obvious case, we take her to be rejecting conventional (typically unstated) ends and asking for an unconventional end. Probably even a selfish end! This is what draws our ire.
To review, both 'good' and 'helpful' have the same meaning, the same definition. And both words have a blank space in this definition where an end needs to be plugged in, but we're much more likely to fill in the blank for 'good' without asking for clarification.
(Frankly, I'm surprised this isn't already a popular catch phase in moral philosophy. Is there something embarrassingly wrong with it? I'll risk it. As Francis Bacon wrote: "[T]ruth emerges more readily from error than confusion.")1
1. From http://en.wikisource.org/wiki/Novum_Organum/Book_II_%28Wood%29