Being “not born of a woman”, in its crudest sense, translates to a modern day cesarean section.
When Macbeth went to the three witches to inquire about his fate from them, they gave him a prophecy and assured him that “no man born of woman shall harm Macbeth”.
Macbeth, concluding that it was impossible for any man to not be born of a woman, went on his way and lived as he pleased.
However, Macduff was “ripped untimely” from his mother’s womb (that means that he was born via cesarean section), so he was the only person able to kill Macbeth.