In the beginning of the play, Hamlet is seeking revenge against his uncle for killing his father. However, as the play progresses, Hamlet starts to question whether or not revenge is the right thing to do. He is torn between his desire for revenge and his moral qualms about killing another person.
Eventually, Hamlet decides that he does not want to kill his uncle and instead chooses to forgive him.
This is a major reversal in Hamlet's character, and it shows that he has grown and changed throughout the course of the play.