What does Circe mean by the Underworld?

In Classical Greek mythology, the Underworld refers to a physical realm in which the dead’s souls existed for all eternity. Separated from the realm of the living, the underworld has several particular characteristics that distinguish it from other conceptions of life after death.
Upon death, a person would be buried with coins on his or her eyes in order to pay fare to Charon, the ferryman who transported one’s soul across the river Styx into the underworld. Other features include Cerberus, a three-headed dog that guards the entrance to the underworld; the god of it all, Hades; and his unhappy wife, Persephone.
When Circe gives Odysseus instructions to travel to the Underworld, she wants him to speak to the soothsayer Tiresias, who is blind. She warns Odysseus of the treacherous journey he must take in order to get to his destination. Circe gives him the tools he needs to conjure the spirits of the dead, including Tiresias, who tells Odysseus that he is being punished by Poseidon for the death of the god’s son, Polyphemus.
Overall, the Underworld is a well-established part of the mythological world of the ancient Greeks. Circe’s power as a sorceress and immortal allows her to connect Odysseus with this realm of death.