In Shakespeare's Macbeth, the witches first use the line you ask about in Act 1.1:
Fair is foul, and foul is fair.
Macbeth then echoes them in Act 1.3:
So foul and fair a day I have not seen.
First, Macbeth is identified with and connected to the witches by his echo. but there is more to the similar quotes than just this.
The line establishes and contributes to the theme of illusion-reality in the play. In Macbeth what seems to be fair is often foul, and even when the fair isn't foul, it may be suspected of being foul.
This is evident at numerous points in the play. I'll mention a few.
Duncan tells Malcolm that there is no way to know what is on a man's mind by looking at a man's face, and that Cawdor, the traitor, was a man he had trusted absolutely.
Duncan trusts Macbeth, also, and Macbeth assassinates him.
Banquo is not in the process of rebelling against Macbeth, although he does suspect him of treachery. Macbeth kills him because he suspects him.
Malcolm is not sure whether or not he can trust Macduff when Macduff comes to England to join Malcolm in a fight against Macbeth, so Malcolm puts him through a series of tests. Macduff passes.
The witches equivocate and deceive Macbeth. They tell him things which sound true, but are deceptively untrue.
The theme of illusion-reality is prevelant in the play, and is introduced and contributed to by the idea of the fair and the foul.
Part of how the line is proven is through the actions of Macbeth and Lady Macbeth. The notion of an inverted set of values is seen when Lady Macbeth begins to plant the seeds of disloyalty and murder in her husband's mind. This is when we begin to see the counsel of husband at the hands of wife not seek to broaden community or nurture, but actually sever and destroy. The inversion of values and of morality is present in such a condition. Throughout the moral depravity of both characters what was seen as horrific and something which could not be done becomes embraced and almost seen as common. Into this moral abyss, Macbeth plunges himself without any frame of reference or detachment because "fair is foul, foul is fair."
This line is the most important line of the first scene. It is the line that is said by all three witches. It is comprised of monosyllabic words. Obviously Shakespeare is calling attention to this paradox. Its application to the witches is quite evident. They are easily associated with evil forces, meeting in "thunder, lightning, or in rain." They associate with cats and toads, and they "hover through the fog and filthy air." So, of course, what seems fair to them may seem foul to others, and vice versa. These witches, we later learn, enjoy creating misery and stirring up trouble.
Interestingly, though, Macbeth echoes the witches in the third scene with "so foul and fair a day I have not seen." This line becomes more complex when uttered by Macbeth. The weather, of course, has been stormy or "foul." But Macbeth has won the battle against Norway. His forture this day has been "fair." Yet, his victory cost the lives of many men, so fair and foul are intermingled. But it cannot be ignored that this phrase also links Macbeth to the witches who had earlier claimed that they were to meet Macbeth on the heath. His words following those of the witches so closely seem to link him with their evil.
This connection becomes even more apparent when Macbeth hears his prophecy, which seems to be one of good fortune and fame. Yet this fair prophecy is undermined by the foul thoughts that leap into Macbeth's mind when he hears that he may become king of Scotland. He thinks the unthinkable: assasinating Duncan. Again fair and foul are intermingled.
With Lady Macbeth the phrase becomes even more layered. She cautions Macbeth to "look like the innocent flower, but be the serpent underneath." For her, fair disguises the foul.
These various applications of the expression can be seen throughout the play, but perhaps it most aptly applies to the topsy-turvy world that Macbeth creates when he disturbs the order and kills Duncan, becoming a tyrannical ruler. In so doing, he turns fair men like Macduff into rebels who must commit treason to restore order.
In the play Macbeth by William Shakespeare, the quote "fair is foul, foul is fair" could be applied to the castle. It sounds like a lovely place - the sort of place where birds feel secure and contented enough to nest in its buttresses, where the weather might always be warm and the atmosphere sweet and welcoming. Welcoming indeed is Lady Macbeth, greeting her husband's guests like the perfect genteel wife and aristocratic hostess. But she is merely putting on a superficial show to gull the guests into a false sense of security and to hide the truth and the real motive. Underneath, everything in the castle is rotten and decaying and the scene for bloodthirsty events.
This line occurs in the opening scene of the play.This statement can mean that what seems fair from one point of view can not be considered fair from another.Foul weather,which is the cause of trouble for others,seems good to them.Macbeth wants to be the king.It appears to him and his wife really good.They murdered the king,Banquo and slaughterd Macduff's family.Macbeth did all this for his own good.thus the foul was fair to him.But this fair was foul for others.
Another interpretation can be this-what looks fair can not be fair(form the point of view of character).Fair lady Macbeth was not good from heart.She only pretends to be good with the king and motivates her husband to kill him.On the other hand Malcolm pretends to be a bad person but was fair from heart and soul..This play of fair and foul can be seen in the whole play,from the very beginning till the end.