Their chant in the opening scene introduces the theme of how to distinguish what appears to be true from what is really true. "Fair is foul, and foul is fair" offers a paradox indicating that we cannot tell one situation from another, which is another way of suggesting the tension between appearance and reality. The witches later seem to appear out of nowhere, causing Macbeth to wonder if they were even there, as does Banquo, when he says "have we eaten on the insane root/ That takes the reason prisoner?" (1.3.88-89). Later in 1.4, as Macbeth plans the murder, he says "Stars, hide your fires; /Let not light see my black and deep desires," this time asking nature to cloak his reality (deep desire to do evil and kill Duncan) in the dark. He reiterates this difference from what appears to be true vs what is true when he says "False face must hide what the false heart doth know" (1.7.95). In all of these cases, Macbeth acts in a way that disguises what he has in his heart, and as for the witches, they repeatedly equivocate on the full truth of the future--the disaster that will happen when he kills Duncan--and in this way hide the reality of the future from him.
My favorite example of this theme is in Lady Macbeth. She 'appears' to be a strong and formidable woman. Macbeth cautions her to "bring male children only" because she is not delicate or feminine.
Macbeth: Bring forth men-children only;
For thy undaunted mettle should compose
Nothing but males. (1.7)
She convinces Macbeth to kill Duncan and plots the way in which is will be done. However, she has a sensitive and fearful side that she hides. She claims that she wouldn't kill Duncan because he looked like her father as he slept. She faints upon hearing that Macbeth killed the guards, and tries to caution him against further action before he arranges Banquo's death. She succumbs to her guilt about the murder in the end, allowing it to consume her to the point of madness and suicide. Clearly, she is not what she seems.