When the Weird Sisters say that "Fair is foul, and foul is fair," they mean that things that seem good could really be bad, and things that seem bad things could really be good (1.1.12). Macbeth recalls this motif when he says, "So foul and fair a day I have not seen" (1.3.39). In one sense, the day has been foul because he has been involved in bloody battles; in another sense, it's been fair because his forces have prevailed.
Then, when the Weird Sisters approach him with their prophecies, their words seem quite good, predicting a noble future for Macbeth; however, Banquo cautions him that
"oftentimes, to win us to our harm, / The instruments of darkness tell us truths, / Win us with honest trifles, to betray 's / In deepest consequence" (1.3.135-138).
Banquo doubts the motives of the witches, and he believes that they might be trying to appear honest in order to mask their manipulative intent and draw Macbeth and Banquo into cementing their own destruction. He is right. It does not take long for Macbeth to consider murder in order to make the prophecy come true.
Further, Duncan laments the way he was deceived by the old Thane of Cawdor, now a traitor. He says, "There's no art / To find the mind's construction in the face" (1.4.13-14). In other words, the old thane seemed like a good and loyal friend, but this facade concealed a disloyal and murderous interior.
After Macbeth arrives home and begins to plot Duncan's murder with his wife, she says to him, "Look like th' innocent / flower, / But be the serpent under 't" (1.5.75-77). She wants him to appear loyal and friendly, as he always has, and use this "fair" exterior to hide the "foul[ness]" of their plot against the king.
Even poor Duncan rides up and declares that "This castle hath a pleasant seat," complimenting the looks of the Macbeths' home (1.6.1). However, the castle's fairness hides the evil plan the couple is hatching to murder him. Moreover, he calls Lady Macbeth a "fair and noble hostess," which, if he knew what foul deeds she was plotting against him, he would never say (1.6.30).