In the "apparition scene," act 4, scene 1, of William Shakespeare's Macbeth, Macbeth goes to the witches in the heath—possibly the same heath where the witches appeared to Macbeth and Banquo in act 1, scene 3 and prophesied that Macbeth "shalt be King hereafter" (1.3.53)—and Macbeth confronts the witches and demands to know what the future holds for him.
Rather than answer his questions themselves, the witches call upon their "masters" (4.1.67), the spirits or Fates that the witches consult for their prophecies, who appear to Macbeth in a series of apparitions.
The first apparition, "an Armed Head," a disembodies head wearing a helmet, tells Macbeth, "Beware Macduff; / Beware the Thane of Fife" (4.1.79–80). Macbeth responds that he's concerned about Macduff, saying "Thou hast harp'd my fear aright" (4.1.82), but Macbeth believes that he can defeat Macduff, so he's not particularly concerned about out the first apparition's message.
Macbeth should have been concerned about Macduff and about the apparition itself—a severed head—which graphically demonstrates Macbeth's fate.
The second apparition appears as "a Bloody Child"—bloody as if from childbirth, not from injuries—who tells Macbeth to "Be bloody, bold, and resolute" (4.1. 87), which Macbeth already believes he is, and that "none of woman born / Shall harm Macbeth" (4.1.90–91). Macbeth takes this to mean that he's invincible, that no one can kill him, including Macduff.
MACBETH. Then live, Macduff. What need I fear of thee? (4.1.92)
Nevertheless, the ever-cautious Macbeth decides to have Macduff killed.
MACBETH. But yet I'll make assurance double sure,
And take a bond of fate: thou shalt not live. (4.1.94–95)
Macbeth seems to think that the second message, that "none of woman born" can harm him, somehow negates the first message, "Beware Macduff." Macbeth later learns that these are two messages are very closely related and that the first message doesn't at all negate the second message.
The third message, from the third apparition, "a Child Crowned, with a tree in his hand," tells Macbeth to ignore his enemies, because Macbeth can't be defeated "until / Great Birnam Wood to high Dunsinane Hill / Shall come against him" (4.1.103–105).
Macbeth decides that this means that he can't be defeated, since no one can "bid the tree / Unfix his earth-bound root" (4.1.107–108) and have the forest march against him at Dunsinane Castle.
Macbeth fails to take notice of the apparition who gave him the third message, the child who was holding a branch of tree "in his hand," which is exactly the manner in which Macduff's army attacks Macbeth at Dunsinane.
The messages of these three apparitions, although important to Macbeth, are incidental to what Macbeth really wants to know. The prophecy that the witches made to Banquo in act 1, scene 3, "Thou shalt get [beget] kings, though thou be none" (1.3.7), still haunts Macbeth, and he demands to know "shall Banquo's issue ever / Reign in this kingdom?" (4.1.114–115).
If the answer to Macbeth's pressing question is "no," Macbeth truly has nothing to fear from Macduff or anyone else. If the answer is "yes," the messages of the other three apparitions are of no real consequence, because whether Macbeth is killed or dies of old age, Macbeth will leave no legacy and Banquo's descendants will become Kings of Scotland.
The witches tell Macbeth not to ask that question, but Macbeth insists on knowing the answer, and he threatens to put "an eternal curse" (4.1.118) on the witches—who, as witches, would seem to be invulnerable to curses—if they don't tell him.
The witches decide to answer Macbeth's question, and a fourth apparition presents "A show of eight Kings, and Banquo last with a glass in his hand." A parade of eight kings, all resembling Banquo, appears, followed by Banquo himself, who holds up a mirror to show Macbeth the many, many more kings who look like Banquo that stretch in a line "out to the crack of doom" (4.1.130).
Macbeth demands to know if this apparition is true, and a witch assures him "Ay, sir, all this is so" (138).
The irony of the message of the fourth apparition is that it's actually untrue. Even though the apparition shows a line of kings descending from Banquo and stretching into infinity, at the end of the play, Duncan's son Malcolm—not Banquo's son Fleance—becomes king. The line of the Kings of Scotland actually descends from Duncan, not from Banquo.