The opening scene of Shakespeare's Macbeth, as all opening scenes do, acts as the exposition. As such, the introduction of the three sisters is significant. They declare, "Fair is foul, and foul is fair"(1.1.12) They are bearded,yet appear to be women; they speak in truths that "betray," so much so that Macbeth,without realizing it, repeats in the next scene what they have said, "So foul and fair a day I have not seen" (1.2.39).
This observation is also a statement of theme that runs throughout the play and casts an evil pall over the actions of the play. The thunder and lightning and sinister words of the three witches also suggest a conflict in the drama of man vs. nature. Of course, good and evil are constantly shifting as "nothing is/But what is not" (1.3.142-143)
The first scene of Macbeth serves as a prologue to the whole Play and Introduces in the form of the Witches (Weird Sisters, weird being a derivative from the old english word "wyrd" which translates into "fate" or "destiny") the forces of action that will control the fate of the different characters. This scene serves also to set the general mood of the play and the setting of it, as well as Shakespeare's use of images in his language (thunder and lightning) Each phrase and word pronounced by the Witches will have its resonance further on in the next scenes.
ACT ONE, Scene I.
An open place. Thunder and lightning. Enter Three Witches.
When sall we three meet again, / in thunder, lightning or in rain?
When the hurlyburly's done, / when the battle's lost and won.
(When the hurlyburly's done: The witches propose to meet again when the battle now in progress is finished - whe nit is lost and won --> lost by one side and won by the other)
That will be ere the set of sun.
(ere the set of sun- "before sunset")
Where the place?
Upon the heath.
(They announce the place where Scene III will be set)
There to meet Macbeth.
(Introducing the character of Macbeth)
I come, Graymalkin!
(Graymalkin is the name of a cat; witches were said to keep cats as assistants in theri ceremonis.
Paddock calls. - Anon! -/ Fair is foul, and foul is fair:/ Hover through the fog and the filthy air. [exeunt
(Paddok - "toad/ Anon! - "(We are coming) at once"/ "fair is foul, and foul is fair" The good and bad are confused; there is an evil influence. This particular verse will be of significance as the rest of the play is unfolded./ "Hover though the fog and the filthy air" imagery used by Shakespeare, along with thunder and lightning to set the place and the mood of the scene.)
As you can see the Witches are useful instruments for Shakespeare to announce what will happen in the play, to slip little pieces of information into the audience: here we are told of a battle taking place that shall end before the setting of the sun with one side winners and the other losers, and most importantly, we are told of a future gathering with Macbeth, which shall take place in Scene III and will be of crucial importance for the rest of the play.