Banquo is Macbeth's loyal general, but Macbeth fears him because the witches' prophecy foretold that Banquo's descendants will become kings of Scotland. Macbeth sees Banquo as a threat to his own throne and decides to have him eliminated.
Fleance, on the other hand, is Banquo's young son. Macbeth also wants Fleance killed because he fears that he may one day seek revenge for his father's death.