The first quote is from the very short Act 1, sc. 1, and the second quote is from Act 1, sc. 3. The first quote is spoken by the witches at the end of the scene. The significance of the quote is that it sets the stage for the play to follow and it sums up one of the prevailing ideas in the play: that appearances will be deceiving ("fair is foul", "foul is fair"). What looks good may be bad and what looks bad, may be good. We see this in one form in the characters of Macbeth and Lady Macbeth. They put on good faces to greet Duncan into their home, all the while planning to kill Duncan. They act like they are innocent, but they are guilty. Later, Macbeth treats Banquo like he is concerned about Banquo attending Macbeth's banquet when Macbeth really is planning a way to kill Banquo. What looked fair, turned out to be foul. The opposite side of that quote is apparent in the second meeting between Macbeth and the witches when they give him prophecies that seem, to him, to show him to be invulnerable. The situation looks positive, or fair, but the truth is that the witches tricked him with semantics and the the "fair" situation turned out to be "foul" when Macbeth is killed by Macduff. The second quote is spoken by Banquo to Macbeth right after Ross tells Macbeth that Duncan has named Macbeth the new Thane of Cawdor, which the witches had just predicted. Banquo seems to know that the witches cannot have imparted good news because they are instruments of the devil. He is astonished that what they said would happen has indeed happened. Typically, the devil and his cronies would lie. Of course, while the witches did accurately predict that Macbeth was to be named the new Thane, they are leading him down a road of ambition that will end with his death.