known as the Three Estates: the Clergy, the Nobility, and the Peasantry.

The clergy were those ordained in the church. Bishops, priests, monks, and nuns belong in this category. In Chaucer's story, there are plenty of clergy on the pilgrimage, though they vary in levels of piety. For example, the Prioress acts more like a noblewoman with her affected courtly manners and elegant clothing; the Monk is more interested in hunting than religious contemplation; and the Friar is a womanizer rather than a servant of the poor. Only the Parson is presented as truly dedicated to God. The corruption among the clergy in Chaucer's story is a reflection of the abuses of power within the church during this period, though the presence of the Parson shows that there were still clergy who took their dedication to God and the poor seriously.

The nobility was generally seen as those who fought. This class included the aristocracy and the royal family. In The Canterbury Tales, the nobility is represented by the Knight and the Squire. Both are men of the fighting profession, with the Knight being particularly highlighted as a man of humility and honor.

Finally, the peasantry were the laborers, though some have elected to call this estate the commons, instead, since it included middle-class tradesmen and merchants. For example, the Wife of Bath is neither nobility nor clergy, but she is a merchant who has done well for herself financially. She might not be able to have the same privileges as the Knight or even the clergy members, but she is certainly wealthier than the Clerk or the Miller, who are also part of the peasant/commons class.

Taken altogether, the pilgrims represent medieval English society in all its diversity of class and philosophy. Chaucer gives the reader vital information regarding the background and character of each pilgrim, showing just how complicated both people and society were in the fourteenth century.

The Prologue is a mirror to fourteenth century English society because in it Chaucer introduces us to various types of people who would have been familiar in that culture. There is an emphasis on the Church or clerical professions, as members of that group would mostly likely have had the inclination and the leisure to go on a religious pilgrimage, but Chaucer goes beyond them to introduce us to a wide spectrum of the contemporary society.

The Prologue introduces us therefore to a cavalcade of people, some familiar to us even today, such as the rich widow, the Wife of Bath. In the fourteenth century, such a woman had the time and money to go on a pilgrimage; today, we can imagine her taking cruises and being similarly entertaining in the stories she might tell, perhaps at the Captain's table at mealtimes. However, other characters, such as the Pardoner, seem much more alien to modern society. A pardoner was a clerical person who granted indulges or pardons (forgiveness) of sins in exchange for donations to the church (of which this fraudulent pardoner keeps the bulk for himself). Other fourteenth century figures that go on the pilgrimage include a knight, a friar, a shipman, a merchant, a man of law, and a physician. What is interesting is that some of these professions are still with us to day, but Chaucer reveals in The Prologue, with its quick character and background sketches, how differently these professions were often practiced 800 years ago. We can learn quite a lot about how life was experienced in this period through reading The Prologue, including the kind of details that might not be included in sweeping historic accounts focusing on kings and battles.

I would note, too, that Chaucer writes in Middle English, not Old English, as the other answer states. Middle English developed after the Norman Conquest, as Germanic Old English and the conquerors' French hybridized to form what was essentially a new language.

You can read the enhanced version of the Prologue here on eNotes. What you will find are setting markers that place it in the 14th century. First of all, a group of people is making a pilgrimage to the shrine of Thomas a Becket at Canterbury. Pilgrimages to this shrine were very common at this time period. Chaucer's group is leaving from Southwark to go to Canterbury. They are meeting at an Inn, the only form of lodging in this time period, and they are walking and/or going on horseback, so you know it is not modern times. There were many inns and taverns in Southwark at this time and it was a popular starting point for pilgrimages.

At the end of the Prologue, the author says that he is going to describe the pilgrims and that he will start with a knight. Again, this is a clue to the time period.

Also, the Old English language in which the Prologue is written is a clue to the time period.

It is often said that Chaucer's "Prologue" to The Canterbury Tales provides modern audiences with a glimpse of life during the Middle Ages:

The “General Prologue” re-creates a lively image of Chaucer’s world. 

Another source notes:

...we are indebted to him for the most vivid contemporary description of fourteenth-century England.

The same source points out:

In Chaucer's day it was customary throughout Europe for members of all classes to travel to religious shrines...By using the device of a journey, it was possible to bring together quite naturally persons of varied occupations and diverse social rank, a rarity in medieval society. Thus Chaucer was able to present in his work a cross-section of medieval society...

This collection included the three groups that represented the prevalent social strata of medieval England: feudal, ecclesiastical and urban.

A "microcosm" is...

...anything that is regarded as a world in miniature.

In essence, the world represented in Chaucer's "Prologue" represents a smaller version of the bigger world. When studying the diverse members of the pilgrimage—people who at that time would never associate with each other except when traveling on a religious pilgrimage—Chaucer provides us not only with those found in everyday society, but he is also honest about some of the problems society faced because of these people. He entertains by pointing out typical human foibles of many of these people; though he does not judge, he provides us with information so that we can decide for ourselves that, for instance, most of the clergy (servants of the church) were crooks, rather than servants of the people. Without these kinds of details, we would not be able to see these individuals (and their shortcomings) so clearly. We also find that people in Chaucer's time are very much the same as they are today. 

The clergy most often miss the important details of their calling. Each was supposed to take a vow of poverty and give all they had to the poor: a Christ-like behavior. The only exception is the Parson. The Monk, the Friar, the Nun, and the Pardoner are all interested in their worldly possessions. One owns a fine hunting horse; another has clothes, jewelry and a dog that eats better than the peasants; another sells pardons stolen from Rome and fake holy relics; two of these men (we infer) get women pregnant and then arrange marriages for them. Only the Parson has given all he has to help the poor, weak and afflicted in his parish. This gives a clear picture as to the struggle the Church had to keep its servants honest and faithful.

However, among all those in the story, there are also those to admire: the Knight has just returned from battle. He is extremely thankful for his safe deliverance home; so much so that without removing his armor, he embarks immediately to pay homage to God at the holy shrine at Canterbury.

On the other hand, the doctor is a quack. The Wife of Bath is a good woman, but has buried several husbands and is looking for another: the inference is that she has a healthy libido, and wore all of her dead husbands out.

Chaucer is considered by some to be the father of fiction—and literature—because of his gift to so clearly describe people he met on his travels who came from all walks of life, reflecting the values and behaviors of the whole world at large.

Additional Source:

Adventures in English Literature, Harcourt Brace Jovanovich Publishers: Orlando, 1985.