On their way to Poland, Fortinbras and his army march across Hamlet's path and Hamlet envies Fortinbras for his vigor in achieving his own goals (which are to acquire a meager plot of land) by using the force by which Hamlet is finding it difficult to muster. After all have gone, Hamlet is chiding himself for his "dull revenge" and he is frustrated that he has not been swifter in avenging his father's death. He feels guilty that he finds it difficult to get this task done after the wrong that was committed against him. He vows to overcome his guilt and in doing so promises himself that his revenge will be bloody.
Fortinbras moves towards his objective and is willing to face death on no stronger evidence that faith that he is right. Such is the strength of his conviction that his troops will gladly follow. He compares his own journey seeking truth before acting and has come to believe that Fortinbras is the better man for it.
It presents truth as a faith, exclusive of evidence... something is true because we believe it to be true. A contemporary example would be the American incursion into Iraq which was based largely on the belief in WMD rather than any real evidence. Like Iraq, Hamlet learns a different perspective in the next act.
His encounter with Fortinbras presents a perspective but that doesn't make it true. Remember, Hamlet only believes that Claudius killed his father, but by the final scene has come to recognize that belief is insufficient to justify killing him. In the final scene, Hamlet executes Claudius for the two crimes he can prove rather than the one he can't.
In a classic demonstration of an eye for an eye, Claudius dies by his own instruments. If the wine and sword are not poisoned, then Claudius, Gertrude, Laertes and Hamlet will live. The outcome is the proof and in that is justice.