In Macbeth, what does "not of women born" mean?

You are of course refering to the prophecies that the
witches give Macbeth in Act IV scene 1 when he returns once again to their lair,
demanding to know the truth of his fate. The prophecies he receives are typically
obfuscated and unclear so that Macbeth is left to draw his own conclusions and is unsure
of their precise meaning. Note the prophecy that the Second Apparition gives him in this
scene:

Be
bloody, bold and resolute: laugh to scorn

The power of man,
for none of woman born

Shall harm
Macbeth.

Ironically, the
apparition goads Macbeth on into ever-further acts of violence with the false hope that
nobody who is born of woman can harm him. Macbeth himself interprets this by thinking
that he is invincible, as he says he has no need to fear Macduff. Of course it is only
in Act V scene 8 that Macbeth learns the truth. As he tells Macduff that he is
invincible because nobody who was borne of woman can kill him, Macduff replies revealing
the true meaning of teh apparitions words:

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Despair thy
charm;

And let the Angel, whom thou still hast
serv'd,

Tell thee, Macduff was from his mother's
womb

Untimely
ripped.

Therefore the
prophecy refers to someone who was born by cesarian, where the mother's stomach is cut
open and the baby is pulled through that way, rather than borne in the normal method.
Macbeth's misinterpretation of this prophecy leads to his over-confidence and his
doom.