Frodo: What are they?
Aragorn: They were once men. Great kings of men. Then Sauron the deceiver gave to them nine rings of power. Blinded by their greed, they took them without question, one by one falling into darkness. Now they are slaves to his will. They are the Nazgul, Ringwraiths, neither living nor dead. At all times they feel the presence of the Ring, drawn to the power of the One. They will never stop hunting you.
Aragorn convinces the Hobbits to hide. They listen to the Wraiths screech in anger at not finding the Hobbits.