Feudalism was a strict caste system, essentially. People were born into one of the categories and, for the most part, could not move up or down between them. The pyramid-like system was topped by the king. The king was the most powerful person and the landowner. Below him came the nobles, appointed by the king himself, who were vassals of the king and managed a specific piece of land. Next came knights, who provided military protection, and, lastly, at the bottom of the pyramid came the serfs, who worked the land for the nobles.
Feudalism was on the decline during Chaucer's lifetime, and The Canterbury Tales is a commentary on what he thought of the system. Many of the clergy, who would've been on the same level as the nobles, were corrupt and self-seeking. Others, who were a part of the emerging middle-working class, like the Plowman and the Clerk, were dedicated and upstanding citizens. Chaucer provided a glimpse of the corruption of those in the higher classes and commented on the success and upstanding emergence of the working/middle class.