The apparitions give Macbeth several prophecies. They tell him that he should be afraid of Macduff, he cannot be killed by a man born of a woman, that he is safe until the forest comes to Dunsinane, and that Banquo’s sons will reign in the kingdom.
Macbeth apparently does not learn his lessons from the chaos the witches have already caused him. In Act 4, he gets some very specific prophecies that he will use to closely guide his actions in the remaining scenes.
The first apparition is an “Armed Head” or a head with a helmet on. This one tells Macbeth that he needs to watch out—for Macduff.
Macbeth! Macbeth! Macbeth! Beware
Beware the Thane of Fife. (Act 4, Scene 1, p. 59)
Macbeth thanks him and begins to ask him a question, but the witches intervene. The second apparition appears. This one is a “Bloody Child” who tells him it’s all right; he can’t be harmed by any man born of a woman.
Be bloody, bold, and resolute; laugh to
The power of man, for none of woman born
Shall harm Macbeth. (Act 4, Scene 1, p. 60)
Macbeth figures he’s fine. After all, wasn’t every man born of a woman? Macbth is confused by the apparent contradiction here. Macduff had to have been born, so he can’t be a danger.
As it turns out, Macduff was born by C-Section, so he was not technically born of a woman. So both prophecies come true in the end.
The third apparition is a “Crowned Child” holding a tree. Macbeth is really confused by this. This child tells him he is safe until Birnam wood moves to Dunsinane.
Macbeth shall never vanquish’d be until
Great Birnam Wood to high Dunsinane Hill
Shall come against him. (Act 4, Scene 1, p. 60)
Macbeth thinks he must be completely safe then. After all, trees can’t move!
The next thing we know, the cauldron starts to sink and we see Banquo’s ghost with a mirror showing lines of kings, Banquo’s issue. This is the line of kings the witches initially prophesized.
Thou are too like the spirit of Banquo. Down!(125) …
A third is like the former. Filthy hags!
Why do you show me this? A fourth! Start, eyes!
What, will the line stretch out to the crack of doom?(130)
Another yet! A seventh! I'll see no more:
And yet the eighth appears, who bears a glass
Which shows me many more…. (Act 4, Scene 1, p. 61)
Macbeth was feeling pretty confident up until this point. He does not see how the other prophecies can say that he is safe, but this one seems to show that Banquo’s sons will indeed be king.
All of the prophecies do come true. Malcolm has his men cut down trees and pretend to be the forest, so it looks like the forest really comes to Dunsinane. Macduff does kill Macbeth. Banquo’s sons, we assume, become kings.