It is the line with which the three witches close the first scene of the play (Act I, scene 1): "The beautiful is rotten, and the rotten can be beautiful" and "Both hover in the fog and the filth of the air." The common meaning is that, in this story, appearances deceive: what seems beautiful may be rotten, and the reverse.
The line has an immediate echo. In scene 3, Macbeth has not yet heard the witches, and he enters saying that he has never seen a day so ugly and so beautiful at once. He uses words similar to the witches' without knowing of the line.
Later, the play shows several times a face hiding something else. Duncan says that a man's character cannot be read from his face, speaking of Cawdor, a gentleman he trusted who betrayed him, and soon after he trusts Macbeth. Lady Macbeth tells her husband to wear the look of an innocent flower, but to be the serpent beneath that disguise. And Macbeth decides that a false face must hide what the false heart knows.
The characters themselves speak of the prophecies in these terms. Banquo already warns on the heath that evil tells small truths in order to betray in great ones, and in Act V, scene 5, Macbeth says that the demon plays with words, lying while seeming to tell the truth.