The first characters we see in Macbeth are the three witches, who are to have a profound influence over events in the play. You can understand their part in the play and their significance better if you focus on the key questions.
The witches meet in foul weather - they speak of thunder, lightning, fog and filthy air. This introduces Macbeth as a dark, dangerous play, in which the theme of evil is central.
Fair is foul and foul is fair."Macbeth (Act one, Scene one, Line 11)
These words appear to contradict each other - it's confusing. Is that what this play is about? Is everything as it seems? The words introduce the idea of illusion and reality.
We then hear Macbeth echo these same words - his first line is
So foul and fair a day I have not seen."Macbeth (Act one, Scene three, Line 38)
Why are the witches' words in Macbeth's mouth? Are we meant to associate him with the witches and their evil? This echo introduces the idea of a prophecy and prompts the question - Is Macbeth in control of his own destiny?
Read through Act one, Scene one and Act one, Scene three again to remind yourself of what the witches say and do.