One of the definitions of archetype is "a perfect or typical specimen." It is not quite the same thing as a stereotype, which is an oversimplification of a type of something. But for Chaucer's Canterbury Tales, character archetype and character stereotype function in nearly the same manner.
A common character in The Canterbury Tales is the knight. His tale is read fairly frequently. It is not by random chance that Chaucer included a knight in his tale. A knight, and thus everything associated with knights and knighthood, would be familiar to Chaucer's readers. Chaucer wrote his tale in 1392, well before the traditional end of the middle ages (1500). Knights and their code of conduct, chivalry, were common knowledge. The reader finds out that the knight has participated in a bunch of crusades, which now automatically make the knight brave, experienced, tough, etc. His tale is about courtly love and how that goes with his chivalric code of conduct. It is a perfect character archetype.
Another character archetype is the hypocritical friar. A friar is supposed to be a man of God. Honest, humble, trustworthy, etc. Friars usually traveled from place to place, unlike monks who stayed at a specific monastery. That means friars usually needed to count on the generosity of others for food and lodging. It was not uncommon in Chaucer's time though for friars to be less than "Christ-like" in their practice. The friar in The Canterbury Tales is no different when the reader learns he frequently accepts bribes. Even his tale is focused on pointing out the wickedness of another member of the group.
The monk is another archetype. He's supposed to be a man of God focused on prayer and work. Much like the friar though, Chaucer paints the monk in a way thought common of clergy during his time -- hypocritical. The monk is clearly not focused on prayer and work. He would much rather hunt, eat, and drink. Basically, I like to think of him in the way that the Robin Hood movies portray Friar Tuck.
Another archetype is the miller. These guys ground grain and were generally strong guys and not generally known for being cordial. In fact at times they were known as cheats. He rudely tells the host that the order of the story telling is wrong and insists on telling his in the wrong order. His tale is fairly insulting to just about everybody.