Epigraphs are like little appetizers to the great entrée of a story. They illuminate important aspects of the story, and they get us headed in the right direction.
The epigraph to the Pardoner's Prologue and Tale is a bible verse from Timothy's first epistle in the New Testament, 1 Timothy 6:10-- Radix malorum est cupiditas. Ad Thimotheum. In English, it means "greed is the root of all evil." The whole idea behind the Pardoner's Prologue and Tale is that it's the Pardoner's imitation of the sermon he gives as he traipses around the countryside hawking pardons, and medieval sermons usually took one or two specific bible verses as their theme. When the sermon was written down, the scribe might preface it with the verse he took as its theme, just as our scribe has done here.