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.