What is the meaning of lines 122-127 of Act 1, Scene 3 of Macbeth?
To put these lines into context, Macbeth has just been named the Thane of Cawdor by King Duncan, just as the witches prophesied. In lines 122-123, he speaks privately to Banquo and asks what he thinks about the other prophecy, the one that says Banquo's children will be kings.
Banquo's response is one of caution:
That, trusted home,
Might yet enkindle you unto the crown,
Besides the thane of Cawdor. But ’tis strange.
And oftentimes, to win us to our harm,
The instruments of darkness tell us truths,
Win us with honest trifles, to betray's
In deepest consequence.
In other words, Banquo thinks it is possible that Macbeth will next become king, as the witches told him. But Banquo cannot shake the feeling that this whole experience is "strange." He is worried that the witches are, in fact, trying to lull Macbeth into a false sense of security: by telling him a small truth, they may actually be leading him to his own destruction ("to win us to our harm"). Banquo therefore acts as the voice of reason in this part of the scene. He wants Macbeth to realize that there may be more to these prophecies than first meets the eye. This also creates a contrast between the two men: Macbeth is happy to believe the witches because it compliments his sense of ambition. Banquo, however, questions their intentions.
On the whole, these lines are also significant because they foreshadow Macbeth's destruction later in the play.
In this scene, Macbeth and Banquo have been visited by the three witches (weird sisters). They have foretold (or suggested, depending upon your interpretation) that Macbeth is the Thane of Cawdor and will become king thereafter. They also say that Banquo will be less than Macbeth but greater. Clarifying this, they say that Banquo's sons will be kings. Macbeth asks them how he might become Thane of Cawdor but they vanish before answering him.
Angus and Ross meet them and Ross tells Macbeth that he is to receive the title Thane of Cawdor. Macbeth and Banquo are shocked that this first suggestion by the witches has come true. Then Macbeth says to himself in an aside that he is now Thane of Glamis and Cawdor. He adds, "The greatest is behind." He means that the latter (the title "behind" the other) is the greater of the two titles; being Thane of Cawdor is better than being Thane of Glamis.
Since the first suggestion has come true, Macbeth asks Banquo if he now has legitimate hope that his sons will become kings. Banquo answers that he is skeptical and asks to speak with Macbeth about it.