In Shakespeare's Macbeth, the main character (Macbeth) receives a prediction about his future in act I, scene 3. Describe this prediction.

At the beginning of act 1, scene 3 of Macbeth, the "Weird Sisters" (the three witches) are gathering to meet Macbeth, which is something they arranged in the first scene of the play:

FIRST WITCH: When shall we three meet again?In thunder, lightning, or in rain?
SECOND WITCH: When the hurlyburly's done;When the battle's lost and won.
THIRD WITCH: That will be ere the set of sun.
FIRST WITCH: Where the place?
SECOND WITCH: Upon the heath.
THIRD WITCH: There to meet with Macbeth. (1.1.1–8)

The witches are already "on the heath" in scene 3 when Macbeth and Banquo show up. The witches have been standing around talking, basically killing time until Macbeth shows up, and one witch shows the others a thumb that she got from a sailor who lost it in a shipwreck; they all seem pretty excited about the sailor's thumb.
When Macbeth and Banquo arrive, they're a little taken aback when they see the witches.

BANQUO: . . . What are theseSo wither'd, and so wild in their attire,That look not like the inhabitants o' the earth . . .You should be women,And yet your beards forbid me to interpretThat you are so.
MACBETH: Speak, if you can. What are you? (1.3.40-49)

The Witches ignore Macbeth's question:

FIRST WITCH: All hail, Macbeth! hail to thee, Thane of Glamis! (1.3.50)

This is not a prediction. Macbeth is already Thane of Glamis.
In Scotland in the eleventh century, the time period in which Macbeth is set, a "Thane" was a nobleman who was permitted by the King to own land in exchange for his military service to the King. (By the way, "Glamis" is correctly pronounced "Glahms," in one syllable.) Macbeth has distinguished himself on the battlefield, and is well known to King Duncan, which gives rise to his ascension to this title.
The second witch speaks up:

SECOND WITCH: All hail, Macbeth! hail to thee, Thane of Cawdor! (1.3.51–52)

Macbeth is confused, since he's not "Thane of Cawdor":

MACBETH: . . . By Sinel's death I know I am Thane of Glamis;But how of Cawdor? The Thane of Cawdor lives,A prosperous gentleman (1.3.74–76)

This looks like a prediction, but it's not.
In scene 2, we learned that the Thane of Cawdor was a traitor to the King in battle, and the King was seriously annoyed about that.

DUNCAN: No more that Thane of Cawdor shall deceiveOur bosom interest. Go pronounce his present death,And with his former title greet Macbeth. (1.3.74–76)

Angus and Ross are going to show up a little later in scene 3 to tell Macbeth that he's now Thane of Cawdor. So even though this is news to Macbeth, it's not a prediction.
Once again, the witches ignore Macbeth's question, and finally get to the point:

THIRD WITCH: All hail, Macbeth, that shalt be King hereafter! (1.3.53)

This is the prediction we're looking for: that Macbeth is going to be King. This might sound like a good thing, except that it sets in motion Macbeth's and Lady Macbeth's extreme ambition, their ruthless, murderous path to the throne, Lady Macbeth's suicide, and Macbeth's death at the hands of Macduff.

Scene 3 of the first act of the play takes place on "a heath near Forres." Three witches are gathered in a thunderstorm, and they anticipate Macbeth's arrival before he first meets them: "Macbeth doth come."
When Macbeth spots the witches, he demands that they speak and tell him who or what they are. Instead of explaining themselves, the witches address him variously as "thane of Glamis," "thane of Cawdor," and Macbeth who "shalt be king hereafter." Banquo, Macbeth's closest friend, asks them why they are saying these things, as they are not all currently true, but the witches simply offer Banquo a mysterious prediction of his own: that he "shalt get kings, though thou be none."
Macbeth demands that the "imperfect speakers" explain further how it is that he can be thane of Cawdor, and indeed how he could ever be king. He knows he is already thane of Glamis "by Sinel's death," but the witches disappear without explaining what they mean by their comments.
However, just as Macbeth and Banquo are discussing the strange prophecies, Angus and Ross arrive to tell Macbeth that he is now indeed thane of Cawdor, just as the witches predicted. This leads Macbeth to wonder whether the final part of their prophecy for him could, indeed, be possible.

In Act 1, Scene 3, the three witches (also known as the Weird Sisters) prophesy that Macbeth will first become the Thane of Cawdor, and then will be crowned King of Scotland. They then claim that, though Banquo himself won't be king, he will be father to a line of kings and, in some sense, will be greater than Macbeth. 
This prediction is important for two reasons. First, it plants the seed in Macbeth's brain that ultimately leads him to murder his way to the throne of Scotland. Second, it alludes to Macbeth's tragic downfall. For example, we learn that, though Macbeth will become king, his reign is doomed to failure, as it is Banquo's line, not Macbeth's, that will last as royalty. As such, through this prediction Shakespeare quickly signals that Macbeth will become king, but that his inevitable failure also looms ominously on the horizon.