In The Prologue, Chaucer shows the reader that some characters are good characters and some are not. His satirical approach would have been instantly recognized in his day, but it is a little harder for us to understand it. In The Prologue, Chaucer describes Pilgrims from many walks of life. In some cases, they are good examples of their social class or job, but in other cases, they are not. In this way, Chaucer is able to point out the flaws in his society in a humorous and satirical way.
A good example of this is the religious class. Chaucer gives us a really good example in the Parson. He is described as a character who tends his flock diligently, not leaving town just to make money. He lives a simple life, and if one of his flock cannot tithe, he will give his own money to assist them. He is a perfect example of a good church figure. Other religious figures do not fare as well in the story. The monk, for example, is criticized for not wanting to remain in his monastery, studying and praying. Instead, his hobby is hunting, and he owns horses (implying he does not follow the requirements of giving up worldly goods that we associate with monks). The friar is also a religious example, but he does not seem particularly religious in his behavior of spending time at taverns, selling pardons, and taking money from unsuspecting widows.
Chaucer also shows us that commoners and tradesmen can be good or bad. By providing the example of the plowman, who works hard from dawn to dusk and is kind to his fellow man and who gives what little money he has to the church, he sets us up to look down upon the miller, who tells dirty stories, cheats people out of money, and is constantly in the tavern. Chaucer uses these good and bad examples in a satirical way to show what is wrong with his society.