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.