In Greek mythology, Cerberus is the three-headed dog who guards the entrance to the Underworld, preventing the dead from escaping. His significance lies in his role as a sentinel of the realm of the dead, symbolizing the boundary between life and death. Cerberus embodies the fearsome nature of death while also protecting the secrets of the afterlife. His depiction often reflects the themes of loyalty and ferocity in the face of threats, reinforcing the notion of guardianship in mythological narratives.