The opening scene of Macbeth is significant for a number of reasons. First, it begins to establish the mood of the play as dark, foreboding, and tense. Notice that the stage direction calls for "Thunder and Lightning," creating a sinister and spooky atmosphere, even before any words are spoken. Further, the fact that the three witches are the first characters we meet adds to this feeling. They reference their animal familiars, often malevolent supernatural beings that were believed to help witches with their magic, increasing our sense of unease.

Second, the opening scene begins to develop the character of the Weird Sisters. They speak in trochaic tetrameter, a rhythm that sounds somewhat menacing and chant-like, especially compared to the speech of most other characters in the play (which is in iambic pentameter, a meter that sounds most like typical speech); they sound otherworldly, as though they are casting spells already. They make their plans to meet Macbeth after the battle, and they provide some clues that their intentions toward him are not benevolent. They say, "Fair is foul, and foul is fair; / Hover through the fog and filthy air" (1.1.12–13). The first of these lines seems paradoxical, for how can something fair be foul, or something foul be fair? It seems as though the sisters are going to make things seem fair, or good, when they are actually foul, and make things seem foul, or bad, when they are actually fair. They are deceptive and mysterious, and it is important that we have some idea of this before we see their encounter with Macbeth and Banquo.

First, the opening scene establishes the main character of the play: Macbeth himself. The audience becomes aware that the witches of the opening scene will meet Macbeth at a battle, and the play assumes an ominous tone. We learn that there will be dangerous conflict when a battle is both "lost and won." Though seemingly a paradoxical statement, this foreshadows that Macbeth will win in his quest to become king but will lose much, including his values, his wife, and ultimately his own life.

The opening act also establishes an element of the supernatural that will be woven into the play; Macbeth will meet with these witches who prophesy the "hurly burley" events to come. This element of the supernatural forces the audience to consider that Macbeth's world is situated on an unnatural order and that there exists a possibility of unnatural evils within the play. This sentiment is echoed in the closing of the scene:

Fair is foul, and foul is fair

A second paradox in such a short scene establishes a setting that is off-kilter. In this play, Macbeth will portray himself as a fair friend to Duncan—yet Macbeth possess a foul heart. Audience members will likely remember this paradoxical line when Macbeth echoes the sentiment in his own opening line:

So foul and fair a day I have not seen.

This line links Macbeth both to the initial ominous predictions of the witches and to the unnatural order that they have established in the opening scene.

The short opening scene of Macbeth—which focuses solely on the bleak, stormy weather and the words of the witches—is important for establishing the dark, foreboding, and eerie mood of the entire play. From the start, we have a world that is amok, out of kilter, and populated by supernatural spirits that appear malevolent. This scene cues the audience to expect a world out of joint and filled with dismal happenings.

Second, the words of the witches that "fair is foul" and "foul is fair" establish the theme of the play from the beginning. This is a play where appearances deceive—most notably the seemingly sweet prize of becoming king of Scotland, which turns out to be foul indeed for Macbeth.

As in much of Shakespeare, for example, Hamlet or Julius Caesar, a world in which spirits are let loose, as the witches are, is one where we can expect bad events to occur—as is the case in this play.

In the opening scene of the play, the Three Witches discuss where and when they will meet again. They decide to meet in an open field after a certain battle takes place, where they plan on meeting Macbeth. In the last lines of the scene, the witches recite the saying, "Fair is foul, and foul is fair Hover through the fog and filthy air" (Shakespeare, 1.1.12-13). The opening scene not only sets the tone of the play but also characterizes the witches and introduces a recurring motif. The audience immediately realizes that the Three Witches have a capacity for evil and will possibly manipulate Macbeth following an unspecified battle. Their statement that "Fair is foul, and foul is fair" indicates that appearances will be deceiving throughout the play. This motif will be repeated and remind the audience that characters, situations, and prophecies are often misleading. Also, the stormy weather creates an ominous, foreboding atmosphere, which again associates the witches with evil deeds. By the end of the opening scene, the audience is conditioned to expect the unexpected and to view the witches with suspicion.

The opening scene of Macbeth helps to set the tone for the rest of the play. The witches, meeting amidst a gathering storm, announce their intention to find Macbeth after the battle. They do not say what their intention is, but it is clear that they mean evil, and the gloomy atmosphere, with "fog and filthy air," only adds to the sense of impending doom. As they depart, they sing a rhyme, "fair is foul, and foul is fair," that foretells the play's focus on deception and appearances, as well as on evil itself. It is a short scene, but it tells the audience much about what is to come.

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As in most expositions, the opening scene of "Macbeth" by Shakespeare establishes a mood.  With the three witches's arrival and portentous remark,

Fir is foul, and foul is fair./Hover through the fog and filthy air (I,ii,10-11)

the motifs of evil confusion and the supernatural are introduced.  This opening scene also introduces the theme of fate as a force in the action of the play.  And, since the Elizabethans were very interested in the elements of the supernatural, this first scene captures the attention of the sixteenth century audience, aristocracy and groundlings alike.