It foreshadows a later event in which he was killed by Macduff who was taken from his mother's womb, meaning he wasn't born of a woman.
The spirits predicted that no man born of a woman could harm Macbeth
Macbeth does not believe that Malcolm cannot harm himself. He is quite prepared to believe that Malcolm might harm Malcolm, which is what "harm himself" means. Macbeth does believe that Malcolm cannot harm him (i.e. Macbeth) because Malcolm's mother was apparently a woman and the witches said that none of woman born could harm Macbeth.
No one born from a woman will be able to harm him
The prophecy said that no man of woman born could harm Macbeth. Macduff, however, was born by Caesarian section, was "from his mother's womb untimely ripp'd". He was not therefore born in the usual sense. Due to this loophole, he is able to kill Macbeth. To be fair, one of the other prophecies warned Macbeth to beware Macduff.
The power of man, for none of woman born shall harm Mcbeth
Because no man of woman born can harm Macbeth. And Malcolm was clearly of woman born (and dead useless in a battle, as we know from the second scene)
"No man of woman born shall harm Macbeth."
The spirits predicted that no man born of a woman could harm Macbeth
Macbeth does not believe that Malcolm cannot harm himself. He is quite prepared to believe that Malcolm might harm Malcolm, which is what "harm himself" means. Macbeth does believe that Malcolm cannot harm him (i.e. Macbeth) because Malcolm's mother was apparently a woman and the witches said that none of woman born could harm Macbeth.
No one born from a woman will be able to harm him
Macduff was not born of a woman because his mother had a C section. The witches said no man born of woman could kill Macbeth.
The prophecy that none of woman born could harm Macbeth. Macbeth wasn't to know that by a technicality, Macduff wasn't "of woman born".
The prophecy said that no man of woman born could harm Macbeth. Macduff, however, was born by Caesarian section, was "from his mother's womb untimely ripp'd". He was not therefore born in the usual sense. Due to this loophole, he is able to kill Macbeth. To be fair, one of the other prophecies warned Macbeth to beware Macduff.
Because of the prophecy that none of woman born could harm Macbeth.
The power of man, for none of woman born shall harm Mcbeth
He keeps reminding himself of the prophecy that "none of woman born shall harm Macbeth."
The second apparitions said it to Macbeth.