Explain the show of 8 kings in Act IV, scene i of "Macbeth."Kindly tell about the kings too. What was its impact on Macbeth?

The reason the apparition shows the eight kings is to bring to fruition the prophecy the witches gave to Banquo.  Remember?  Banquo will not be king, but his children will, and so will his children's children.  This is the whole reason Macbeth wanted Banquo AND Fleance (his son) to be killed when he arranged the murderers to go after them.

The last king in the apparition holds a mirror to reflect a never-ending line of kings descended from Banquo.  They all look like Banquo, too.  By the way, this play was written for King James I of England who was supposedly descended from Banquo.  This is why the character of Banquo is shown as such a good and honorable person, and why the never-ending line of kings is shown...King James I being one of them.

Macbeth's reaction?  He is furious, of course.  He does not want to be deposed.  He is angry that Fleance escaped his murderers' grasp.  It makes him uneasy to think that his own children will not ascend to the throne and that their will be no Macbeth Dynasty in Scotland.