The bloody child tells Macbeth, "No man born of woman shall harm Macbeth." This prophecy comforts Macbeth despite the first apparition's telling him to beware Macduff. The witches are equivocating here, as Macbeth discovers in Act 5, scene 8. The bloody child's statement has a double meaning.
Macbeth tells Macduff, "I bear a charm'd life"; no one can kill him. To his shock, Macduff declares he was from his "mother's womb untimely ripp'd." Macduff was not born in the normal way; rather, he was born by Caesarean section: his mother's abdomen was cut open and the baby was removed from her body. Therefore, Macduff will be able to kill Macbeth.
At first Macbeth refuses to fight Macduff, but when Macduff threatens to humiliate Macbeth by parading him so that the "rabble" can taunt and curse him, Macbeth chooses to fight to the death.