The “weird sisters” tell Macbeth that he will be thane of Cawdor and King.
When Macbeth meets the three witches, he is the thane of Glamis. It is a worthy title. He is already a nobleman. However, the witches have a prophecy for him. They tell him that he has some much more interesting titles in store. This is how the witches greet Macbeth.
The first title is the one he already has. He already is the thane of Glamis. There was until recently another thane of Cawdor. Macbeth just killed him in battle. There is a king now, King Duncan. Macbeth does not have either of these titles, and yet they greet him thus.
Banquo watches Macbeth’s reaction, and asks him why he is afraid of the witches and seems entranced, “rapt,” especially when what they are saying sounds great. The sisters turn their attention to Banquo, telling him he is "Lesser than Macbeth, and greater.” His sons will be kings, though he won’t be.
Macbeth seems to get hold of his senses around this point and begins interrogating the sisters.
Stay, you imperfect speakers, tell me more:
By Sinel's death I know I am thane of Glamis;
But how of Cawdor? the thane of Cawdor lives,
A prosperous gentleman; and to be king
Stands not within the prospect of belief,
No more than to be Cawdor.(Act 1, Scene 3)
Naturally he wants to know what anyone would want to know in this situation: Where are you getting your information? Of course, the witches do not tell him. They just disappear.
Banquo decides their best bet is to just laugh off the whole business. However, Macbeth is an ambitious man. He can’t let it go. The thoughts of a higher title, and the thought of being king, sound pretty good to him. Things get much more interesting when one of the predictions becomes true. Ross tells him he has a great honor, and addresses him as the thane of Cawdor. Both Banquo and Macbeth react.
They both wonder if it is true, but it is. Apparently, if you kill the guy, you get his title! Well, Macbeth behaved valiantly in battle, and Duncan wants to reward him.
This gets Macbeth thinking. If one prophecy came true, what about the next? When Duncan names his son Malcolm his heir, Macbeth is a little annoyed. Most people would likely think that it would be a perfectly natural thing to do, and on any other day, Macbeth would probably have just clapped Malcolm on the back. Now though, he is angry. He wanted that prophecy to come true, and he feels passed over.
Here we have a story of ambition, greed, and the power of suggestion. The witches planted a seed in Macbeth’s mind, and just stepped back and let it grow. There is no reason in the world for Macbeth to be king, since he is not next in line for the throne. Malcolm is the king’s son, and the most logical choice to be named heir. Also, the king is not near death or stepping down. Before seeing the witches, we had no reason to think of Macbeth as anything but a loyal soldier and subject. Now, suddenly, he has some bloody plans for the king!
When Macbeth encounters the three witches in Act I of Macbeth, they greet him openly by calling him the thane of Glamis (which he is at the time, so this would be of no surprise to him), thane of Cawdor (whom he knows as a fellow thane, so he thinks this cannot be), and he "that shalt be king hereafter" (meaning that in the future, he will be king of Scotland).
Let's look further into each of these titles:
First, in the 11th century Scotland (the setting of Macbeth), a thane was a person of high status often due to military service to the king. Since Macbeth is the thane of Glamis when the play opens, the reader knows that he is respected by King Duncan and has been looking after that area of Scotland known as Glamis for Duncan since the time of Macbeth's father's death. This latter part we learn when Macbeth says in Act I, sc. iii, ln. 71,
"By Sinnel's death I know I am the thane of Glamis..."
Of the three titles with which the Witches greet Macbeth, this first one is the only one he actually is at the time of the meeting.
Now let's move on to the second title...
Macbeth knows that there is already a thane of Cawdor; in fact, he says in lines 72 and 73 of this same scene,
"the thane of Cawdor lives / A prosperous gentleman."
For some readers and audience members, the thane of Cawdor and another character named Macdonwald--both mentioned in previously in Act I--often get confused with one another. There have been TWO traitors to Scotland, one Macbeth killed ruthlessly in battle as reported by the bleeding Captain in sc.ii. This traitor who dies by Macbeth's sword is named Macdonwald, a different man than the thane of Cawdor!
Ross reports to Duncan in Act I, sc. ii that the Thane of Cawdor is an additional traitor--and the thane of Cawdor, unlike Macdonwald, is at this moment very much alive. Duncan says that he wants him executed for his treason and demands that he is found and killed. His death will be reported back to Duncan in sc. iv. Therefore, when the three Weird Sisters call Macbeth the thane of Cawdor, this is very confusing to Macbeth because he knows the thane of Cawdor is alive but doesn't know that he is a traitor. He cannot foresee why he would be granted the title of another thane.
Lastly, when the witches greet Macbeth with the news that he will be king at some point in the future, this too is unbelievable. At this point, he is a good thane and war hero who fought well against the Norwegians, but Duncan is a good and able king. He is very taken aback by this news. You will likely find that this title is the one that causes the most angst for Macbeth as the play proceeds.
As you continue to read, remember to think back to these three simple titles because they play a tremendous roll in the play.