Ghost, Scene five

Are ghosts real? What could they possibly be trying to tell us? In Hamlet scene five he talks to his ghost father. But what could he possibly be talking to his father about? They talked about his death, and how he died. Now if its your first time reading hamlet like me you might have som problems understanding him when he talks about how he died. "With juice of cursed hebona in a vial". In other words he was givin piosoned. This is what he told to his son, as far as we know it was the queen that gave him the poison.