Answer:
The witches (also known as the Weird Sisters) predict that
1. Macbeth will become Thane of Cawdor (he's already Thane of Glamis)
2. Macbeth will become the king.
3. Banquo will not become king, but will have descendants who will.
4. Banquo will be "Lesser than Macbeth and greater." (1.3.68) A possible interpretation of this is that he will have less power (since he won't be king), but he will be greater because he's a better person. *SPOILER ALERT* Macbeth kills King Duncan.
5. The witches also predict Banquo will be "Not so happy, yet much happier."(1.3.69) This could mean that he won't be particularly happy, but that Macbeth will suffer great misfortune, meaning Banquo will be happier.
The messengers, Ross and Angus, come to tell Macbeth that he has been made Thane of Cawdor by King Duncan. This helps convince Macbeth that the witches' prophecies were real.
