The other posts have given you some really great, accurate discussion of Macbeth, the witches, and this scene. I'd like to add just one more idea to their already tasty mix--Macbeth as a target.
This position assumes the witches know what they're doing and deliberately seek out Macbeth in order to further his destruction. While they demonstrate no "real" power in this scene (except, perhaps, their disappearance, which can be explained by the weather and their surroundings), they do use the power of suggestion to lure Macbeth into a web of ambition and murder.
They may wish for Duncan's death, or Macbeth's demise, or chaos in the country caused by the death of a king; in any case, though, they appear to have chosen Macbeth as the agent by which their desired change will happen.
It works. He is their target; and, since all those things happen, their aim is true.
I can understand your confusion. The first scene is mostly about the witches. But they are planning to meet Macbeth when "the battle is lost and won." From the beginning, the witches establish a connection with Macbeth. Macbeth even echoes their most important line "Fair is foul and foul is fair" when he is introduced: "So foul and fair a day I have not seen."
So what do we have here? We know that there will be winners and losers in the play. We know that the witches are planning to meet Macbeth. We know that the witches are evil. We know that Macbeth's military background will come into play. We know that things will not be as they seem.
How do these ideas relate to Macbeth? Will Macbeth be a winner or loser? Will Macbeth's meeting with the witches result in a bargain with the devil? We know that Macbeth is engaged in a battle that is "lost and won." We know that the meeting between Macbeth and the witches will impact Macbeth.
So what the reader can deduce from the opening scene is that Macbeth is engaged in battle, that he will meet the witches, that he may not be able to deduce what is fair from what is foul,and that Macbeth might possibly have a dark side that lures him to the witches.
Macbeth's is a characterization in absentia in the expository scene of Shakespeare's tragedy Macbeth. The characters we directly encounter in the scene are the witches and it is the supernatural instead of the human world that seems to be foregrounded at the beginning. But nevertheless there is an indicative opening onto his character. The witches have come together to preplan their meeting with Macbeth. This is a proof of his significance to the thought of the witches. Macbeth is also commented upon implicitly by the witches e.g. the word 'hurlyburly' is Macbeth's psychic index as his mind is a moral hurlyburly. He is the one who wins and loses the battle at the same time. He wins the apparent political battle but loses the moral battle within. He is also the person who illustrates most aptly the maxim about the mutual convertibility of the fair and the foul. We are just about to see how the warrior hero in the name of the fair Macbeth is to be left suspect with the foul temptations of the witches in act 1 scene 3.
This is a great question because Act I, Scene 1 is so short and obtuse. You need to pay attention to what the witches are saying. The witches are discussing the fact that the civil war will end that particular day and when will they meet again, "In thunder, lightning, or in rain?" They are discussing the fact that the civil war will be "lost and won". I think they are saying that even though the civil war is lost by the enemy and won by Macbeth or Scotland, it is also Macbeth's battle personally that will be lost and won. The witches plan to meet him on the heath. After that they say, "Fair foul, and foul is fair: Hover through the fog and filthy air." Shakespeare has been known for his double meanings (Romeo and Juliet being a good example of that) and this passage would be another example. They are saying that what would be fair is bad and what would be bad will be fair. Along with that they use fog as a cover for it and when they say "filthy air" again, it has a double meaning. What is bad will be covered up by something that looks good, but is filthy or bad. This entire passage tells me that the witches believe that Macbeth seems like a good man on the outside, but really isn't very nice.
nice question this is -macbeth introduced in the play first time in the battle field with banko against macdufhall and norwayan king sweno.