Answer is as follows:
‘So foul and fair a day I have not seen" The first words of Macbeth are related to the witches words. ‘Fair is foul, and foul is fair”.
This line is “significant” because he won a battle, which is good, but the lives lost (death) isn't wonderful.
These words are called “dramatic irony” because the Three Witches use this phrase in the opening scene of the play.
