Okay, so basically what the excerpt is talking about is that Macbeth says that usually when you kill a man, he stays dead, and doesn't come back to haunt his murderers. However, here, in this play, Macbeth constantly sees the ghosts of his victims who come back to torture his already frail psyche. These lines are important because they show how Macbeth is slowly becoming crazy, seeing things, becoming devoured by guilt.
