When Macbeth visits the weird sisters and demands to know whether or not Banquo's heirs will become kings, the witches conjure a vision of eight kings, the last of which holds a mirror that reflects many more such kings. Cool vision, right?
Not to Macbeth. See, these are Banquo's heirs, which means that Macbeth's sons aren't going to become king which means Macbeth had better watch his back.
But it would have been pretty cool to Shakespeare's audience, because, as the stage directions tell us, the last king is carrying "two-fold balls and treble scepters" (4.3). These two balls (or orbs) are a symbolic representation of King James I of England (VI of Scotland), who traced his lineage back to Banquo. At James's coronation ceremony in England (1603), James held two orbs (one representing England and one representing Scotland). It looks like Shakespeare has just paid a nice little compliment to his patron.