The best definition for poor in the passage is meager.
In this scene, the ghost of King Hamlet and the young Hamlet interact, with the ghost telling Hamlet how he was murdered by his own brother, who than took his crown and position. He than asks Hamlet to take the revenge on his uncle for murdering his father and marrying his mother. Poor is used as to mean that his request is simple, not a big deal.
According to Aristotle, the metrical arrangement of words refers to Diction.
