When Odysseus and his men sail to the underworld, they are trying to summon and meet the ghost of the prophet Tiresias. From him, they are going to try to get advice about how to get back home. This happens in Book XI.
Previously, in Book X, Odysseus had gotten advice from Circe about how to get home. She told them that Tiresias would know the best way for them to manage this. Because she tells them this, they go to the land of the dead and start to make sacrifices. The sacrifices that they make attract many, many ghosts but Odysseus holds them off and will not let them get at the blood. He only wants Tiresias for the advice he can give. Later on, Odysseus meets many other ghosts, including his own mother. But the whole point of the sacrifices is to summon Tiresias.