What is Good?
Two years ago, on Reddit, a user named wanderingbishop wrote a fantasy short story that addresses this very thing from the perspective of Garg the ogre. One day, Garg finds an intelligent greatsword who shows him what Good is.
Out of respect for copyright I will not post the story here. I will, however, repeat the link and encourage you to give it a read. It’s one of the better explanations I’ve ever seen that can be given without requiring philosophy classes and argument. And is a very good story.