And we three will go slay this traitor Death He shall be slain who's stopped so many a breath What is the significance of these lines?

These lines (from Chaucer's The Canterbury Tales) show the amazing arrogance and pride of the revelers, and let the reader know that the three companions are about to embark upon a foolish quest. When they do meet death, they don't recognize him, and he sees how greedy they are, and directs them to a quantity of gold, for which they kill each other.