After the witches tell Macbeth and Banquo the prophecy (that Macbeth will become Thane of Cawdor and later king, and that Banquo's children will becomes kings), they run into Ross and Angus. The men give news that the King has decided to make Macbeth Thane of Cawdor. This is good news, of course, but it also shows that the prophecy might actually be true.
The two men react differently. Macbeth believes the prophecy and asks Banquo if he wants his children to be kings. Banquo distrusts the witches and comments that the devil often tells half truths. The two men agree to speak on the prophecy later in private. Macbeth is already wondering if he will become king naturally or if he will have to take the throne through dark deeds.The split reaction shows a difference in character between the two men. Macbeth, overcome with ambition, can think only of his chance at power, while Banquo is much more concerned about how the witches might deceive him into committing evil deeds.