This quote is spoken to Macbeth from the Third Apparition, a crowned child. Macbeth has asked the witches to shed some further light on possible dangers to him after he has killed Banquo and Macduff’s family. He is told that he cannot be hurt until a forest comes to his castle. The prophecies seem ridiculous, so Macbeth feels safe.
Macbeth visits the witches for the second time in Act 4, because he is interested in knowing what more they have to tell him about his future.
I conjure you, by that which you profess,
Howe'er you come to know it, answer me:…
Even till destruction sicken, answer me
To what I ask you. (Act 4, Scene 1, p. 59)
Macbeth has come full circle. He was leading a pretty normal life until he happened upon the witches, and he ended up king. Now he is ready to find out if he is going to stay that way.
He is greeted by “a Child Crowned, with a tree in his hand” (4:1, p. 60).
Be lion-mettled, proud, and take no care
Who chafes, who frets, or where conspirers are:
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 is thrilled. The witches have told him he is safe until the forest storms his castle. Since forests do not generally move, he is giddy with relief.
That will never be.
Who can impress the forest, bid the tree
Unfix his earth-bound root? Sweet bodements, good! (Act 4, Scene 1, p. 60)
Of course, the prophecy comes true. Malcolm decides that the best way for the army to sneak up on Macbeth is to hide in the forest.
Let every soldier hew him down a bough,
And bear't before him: thereby shall we shadow
The numbers of our host, and make discovery
Err in report of us. (Act 5, Scene 4, p. 83)
Malcolm is aware that anyone who sees a line of trees coming for the castle will be scorned and scoffed at. That is exactly what happens.
Macbeth’s visit to the witches in this scene demonstrates that he is losing control. Hecate admonishes the witches for overstepping their authority in messing with him, and then proceeds to step up the game by making Macbeth even more prophecies. She realizes that he has become dependent on them, and they can really have some fun now.
The crowned child with the tree in his hand represents Macbeth’s throne, and the threat to it. His kingship and his line is limited, as is shown by the prophecy of Banquo’s line inheriting the throne. Taken together, the prophecies show real danger.
The prophecies relieve Macbeth’s concerns, when they should have the opposite effect. All of the suggestions seem so absurd that Macbeth dismisses them. He considers himself safe. The witches get their way, because rather than being forewarned, Macbeth is going to be blindsided. He does not see the real warning there. Every one of the prophecies comes true, and Macbeth is defeated.