What does Hamlet learn from the ghost's speech?

The Ghost confirms that he is Hamlet's father, saying that he has been consigned to hell because he was murdered without a chance to confess his sins. When he demands that Hamlet avenge his "foul, strange and unnatural" death, Hamlet agrees, listening with increasing shock as the Ghost tells him that Claudius murdered him while he slept, pouring poison into his ear. As the dawn begins, the Ghost urges Hamlet to take revenge on Claudius but to leave Gertrude alone. The Ghost then disappears, urging Hamlet to remember him.