So Hamlet is on his way to England, and more specifically, he's on the Danish coast near the Danish castle.
Hamlet sees the armies of Prince Fortinbras of Norway, who it seems has decided to wage war with Poland instead of with Denmark.
Hamlet asks about the cause of the war and is told that the huge conflict is being waged over a tiny, unimportant piece of territory.
Hamlet thinks this is petty but, he concludes that, if Fortinbras can lead 20,000 men to their death over a little patch of land, then he should be able to rain vengeance upon the man who has killed his father and "stained" his mother?
Hamlet swears that he will follow Fortinbras's example, stop delaying and finally carry out his revenge.