The witches are giving Macbeth his fate. All the while he is not interested in his fate but only in his downfall. Another thing that was shown in this scene is the fact that the vision also only appeared when it recurred in Macbeth's mind. Macbeth's mind is so clouded by the fact of being "Invincible" that it almost turns him into a ghost, who sees only power and killing. This quote could interpret either way as a ghost or a vision, whether literal or physical