Cerberus is a creature from Greek mythology often depicted as a dog-like creature with 3 heads. He welcomes all souls into the underworld, but will not let them leave.
He can also be thought of as a metaphor for the ills of our world such as sin (pride, envy), modern-day death culture (war, allowing people who are thought of as inconvenient or weak to die or be killed, etc.), and materialism that says only tangible objects matter when all things are passing except that which is spiritual. These are all things that can welcome a person to Hell and keep them there as Cerberus does.