At the time that Geoffrey Chaucer wrote The Canterbury Tales, between 1387 and 1400, there were three "estates," roughly equivalent to social classes, which had been firmly established in England during the Middle Ages.
The First Estate is the clergy and those who assisted the clergy in teaching and religious activities; the Second Estate is the nobility and aristocrats; and the Third Estate is the peasants or commoners. In terms of feudalism, the Second Estate (nobility and aristocrats) were those who lived in the castles and defended the castles, the Third Estate (peasants and commoners) were those who worked for those who lived in the castles, and the First Estate (clergy) were those who taught and religiously ministered to those in the Second and Third Estates.
What is sometimes referred to as the "Fourth Estate" is the more rural, outlying commoners and anyone who didn't fit neatly into any other category. (In modern times, the "fourth estate" is the independent, "free" press and other media, which stand outside the other three estates.)
In the later Middles Ages, when Chaucer was writing The Canterbury Tales, the three estates were beginning to fragment. For example, there was an emerging class of intellectuals and clerks (like Chaucer himself) in the clergy who studied literature and writing but who didn't perform or assist religious rites. A growing merchant class was also exerting influence throughout England and Europe.
Originally considered simply as an introduction to The Canterbury Tales, scholars now believe that the "General Prologue" stands on its own as an "estates satire," in which Chaucer satirizes each of the three estates through individuals who represent each estate.
The primary focus of Chaucer's satire throughout the "General Prologue" and the "Tales" is the First Estate, the clergy, represented by the Prioress, a Nun, three Priests, a Monk, and a Friar. He also satirizes upper level members of the Third Estate and the emerging intellectual merchant classes, including a merchant, a lawyer, a wealthy landowner, a haberdasher, a sheriff, and Chaucer himself.
Notably absent from the book are members of the guilds (who make an appearance at the end of the procession but about whom no stories are told) and the "high nobility" (kings, queens, other royalty, and members of the ruling aristocracy), although a few members of the "petite," or lower, nobility (the Knight, Squire, and Yeoman) are given their own "tales." Chaucer provides no explanation as to why representatives of these classes aren't included in his book.
Although his satire is generally well-placed, Chaucer is sometimes an "unreliable narrator," meaning that he's a narrator who the reader can trust regarding the truth of the facts in the narration, but who the reader can't necessarily trust regarding the narrator's often subjective interpretation of those facts. This unreliability is what accounts for much of the humor and satire in The Canterbury Tales.