This represents the greed and power Macbeth gained. But the isolation that he gained. His wife died and no one wanted to be around him. He was left with dealing with the guilt eating at his soul.
Lady Macbeth is sleep walking, while a servant and doctor watch her. She rubs her hands together vigorously, that represent the blood she is trying to wash off. This represents the guilt that she feels.
The three witches within the story foretold the story for the audience. They saw visions of Macbeth becoming King, and Banquo's desendants becoming future kings