For this one, you need to look at what they say to Banquo in Act 1, Scene 3:
Thou shalt get kings, though thou be none.
Though Banquo will never be king himself, he will "beget" (the word which "get" comes from) kings - he will father kings. And Fleance, as his son (the only son of his, we presume - and certainly the only son we see in the play) will presumably be a king.
We don't know how it will happen, how Fleance will wrest the throne from Macbeth, or when it will happen. But the witches' prophecies suggests that it actually will happen. So when Fleance escapes from the scene of Banquo's murder, never to appear again in the play, it's a fair assumption that he's eventually on his way to the throne. "Then comes my fit again", says Macbeth, aware that he hasn't removed the key problem. Understandably, he's worried.
The real logic behind Fleance as king is more contxtual. James I, on the throne when Shakespeare wrote Macbeth, was historically a descendant of Banquo's. So Banquo did beget kings - one was probably sitting in the theatre!
Hope it helps!