What is the second witch prophesying when she says ''the pow'r of man/ for none of woman born shall harm Macbeth"?
Your question is a little out of context. Here's the whole passage so you can see that who actually speaks these lines is an Apparition (a ghost) conjured up by the First Witch.
He will not be commanded: here's another,
More potent than the first.
[Thunder. Second Apparition: A bloody Child]
Macbeth! Macbeth! Macbeth!
Had I three ears, I'ld hear thee.
Be bloody, bold, and resolute; laugh to scorn
The power of man, for none of woman born
Shall harm Macbeth.
Then live, Macduff: what need I fear of thee?
But yet I'll make assurance double sure,
And take a bond of fate: thou shalt not live;... (IV.i)
The Apparition conjured by the First Witch is telling Macbeth to laugh with scorn at the power of mankind to overcome him and that he cannot be harmed by anyone born of woman ... which leads him to think he is invincible because everyone must be born of woman. Of course, though, there is someone who can harm Macbeth ... someone who was "untimely ripped" from his mother's womb ... Macduff!
Check the links below for more information about this prophecy and about the later scene between Macduff and Macbeth.