Iambic Pentameter, Rhyming Couplets, Sermon Form
The Canterbury Tales, including our Pardoner's tale, is written in iambic pentameter in rhyming couplets. Every two lines rhyme, and there's a heavily stressed syllable following a syllable with less emphasis: dah DAH, dah DAH, dah DAH, dah DAH, dah DAH. Each "dah DAH" is called an iamb, and there are five per line. Shakespeare wrote in iambic pentameter, too.
For a few hundred lines at the beginning of his Tale, the Pardoner delivers a mini-sermon on the "Tavern Sins" of gluttony, drunkenness, and oath swearing. He's using a typical medieval style of sermon that involves "dilating," or expanding, on a specific topic by citing biblical and literary authorities that relate to it. When the Pardoner begins his mini-sermon on drunkenness, for example, he tells the Biblical story about how Herod was drunk when he ordered the beheading of John the Baptist, then follows this example with a quote about drunkenness from the ancient Greek philosopher Seneca. The whole point of the sermon style is to add to knowledge about a topic by collecting what lots of auctorites, or authorities, say about it.