The ending of Macbeth is very significant to the action of the play given that the ending provides a lesson in morality.
Over the course of the play, readers come to find that Macbeth's demise is the result of his growing ambition. This ambition far surpasses his initial hold to the importance of right over wrong. During the play, Macbeth's growing ambition changes him from a honorable and recognized hero to a dishonored and despised man.
The ending of the play, therefore, offers readers with two closures (regarding the significance of the ending). First, the reader finds closure in the fact that Macbeth must face the consequences for his actions. Second, the ending provides a lesson readers are sure not to forget: that one's holding of good over evil will insure one's survival (as proven by Macduff and Malcolm's survival).
In the end, Macbeth's actions speak much louder than his words. If he would have simply allowed "chance to crown him" he would not have "found" himself in the predicament at the closing of the play.
The final scene of Macbeth is significant for a couple of reasons. Firstly, remember that back in Act IV, Scene I, the witches told Macbeth that "no man of woman born" could harm him. This is important because in the closing scene, it is revealed that Macduff is not of woman born. He was, in fact, "ripped" from his mother's womb which means that he was delivered by caesarean section. So, in one sense, the witches final prophesy has come true: Macbeth's foe, Macduff, is really not of woman born. Rather ironically, Macbeth failed to realize that such an outcome might be possible.
Secondly, the closing scene is significant because it demonstrates the strength of Macbeth's ambition and desire for power. Even though he knows he cannot defeat Macduff, he will not give up the fight. One of the closing images of the play is Macduff carrying Macbeth's head. This is symbolic of Macbeth's fight to the bitter end and a lesson from Shakespeare that naked ambition leads only to one's own destruction.