What is the significance of the show of eight kings given to Macbeth?

The significance of the parade of eight kings revealed to
Macbeth in Shakespeare's Macbeth Act 4.1 is that it demonstrates
that Banquo's heirs will rule Scotland, not Macbeth's.  Ultimately, Macbeth will fail. 

Once Fleance escapes his would-be murderers in Act 3,
Macbeth cannot create the dynasty he is hoping to.  Though he succeeds in having Banquo
killed, Macbeth fails to have Banquo's heir killed.  This is the beginning of the end
for Macbeth, and possibly the climax of the play.  The parade of kings highlights
Macbeth's failure.

It also contributes to Macbeth's fear
and anxiety, and to his further loss of control.  And it highlights the oppositional
elements in Macbeth's mind that conflict throughout the last three acts of the play: 
since his loss of control at the feast when he "sees" Banquo's ghost, Macbeth ping pongs
back and forth between wanting to believe he is indestructible as the visions he sees
seem to predict, and is knowing that the predictions are too good to be true and that he
is doomed. 

The parade of kings also, by the way, gives
homage to the king that the play was produced for:  James I, who was a descendant of
Banquo.