This is the tale that the Pardoner tells on the way to Canterbury. Nothing complicated about that. In the medieval Catholic Church, higher-ranked clergy members like popes and bishops might sign or affix their seals, or unique stamps, to documents that guaranteed a lesser amount of punishment for sins, to the recipient. Pardoners were the people who traveled around selling these documents (and by extension, forgiveness) to people. Many pardoners also did a secondary trade in relics, or physical remnants of the bodies, clothing, or possessions of holy people, which some medieval Catholics believed could have special miraculous powers. Kind of like a piece of Elvis's shirt that a fan might buy on eBay. As part of their sales pitch, pardoners often preached sermons to inspire repentance in their audience, then they'd pull out the goods to sell. The tale of Chaucer's Pardoner is actually the sermon and short story he uses to encourage people to spend money on his pardons and relics.