What does "no man born of a woman would have the power to hurt Macbeth" mean?

This is one of the three additional prophecies the three witches offer to Macbeth in Act VI, scene 1. It is meant to confuse Macbeth, and give him false confidence that he will triumph over his enemies.

Showing him the apparition or vision of a bloody child, the witches tell Macbeth that no man born of woman will be able to hurt him. Macbeth takes this at face value, thinking it means that he is safe from harm. After all, all men are born of women, so it seems nobody can lay a finger on him.

However, the witches are speaking in riddles. What they really mean is that a man born from a Ceasarean section will hurt him. This does seem to be splitting hairs about birth in an unfair way, as one could argue that even a child born by Caesarean is still born of a woman, but the witches at this point actively want to harm and trick Macbeth. They are therefore defining "birth" only as going down the birth canal and being born the natural way.

Th witches are becoming cruel to Macbeth and out and out trying to destroy him. This is because Hecate, the head or "mistress" of the witches, appears in Act III, scene 5, and angrily scolds the three for having prophesied to Macbeth about becoming king without consulting her, and moreover, having done it out of an attempt to help, not hurt Macbeth. She says he is just as bad as other humans, thinking only about himself. He is spiteful and angry and doesn't care a bit about the witches:


And, which is worse, all you have done
Hath been but for a wayward son,
Spiteful and wrathful, who, as others do,
Loves for his own ends, not for you.

Therefore, Hecate tells them to step up their game and deliberately confuse Macbeth, which is what they do. They follow her command that they:


Shall raise such artificial sprites [visions, apparitions]
As by the strength of their illusion
Shall draw him on to his confusion.