Chaucer's General Prologue does several things: it creates both a natural and human setting for the work; gives a description, one by one, of people, or types, who represent the England of his time; and says this diverse group will pass the time during their stopover by having a contest of narrative skill.
These people, in the freshness of April, are going on a pilgrimage to the burial place of a martyr, Thomas Beckett, who, 200 years earlier, was assassinated by the king's agents. Right away, this tells us something about the English people, before Chaucer gets into their specific kinds of employment and their roles in society. They honor the past and tradition. They are religious, at least outwardly—but admittedly, everyone in Europe was outwardly religious at this time. They also are a people able to establish camaraderie with each other. It's a microcosm of human brotherhood bonded by the Narrator's (and therefore Chaucer's) good-natured, friendly, and conversational tone.
The cross-section of society Chaucer shows us is one that demonstrates diversity of position in the kind of work the people do, but it also shows the extent to which this was a time of transition in thought and activity. The Knight can be said to represent the past. The Miller is an emblem of the working class and (as we eventually will see in his tale) the rougher, more down-to-earth side of life. The Pardoner represents religion, but in a way, the portrayal of him shows the growing awareness among people of hypocrisy in the church establishment. The Merchant is the rising middle class. The Prioress and Parson are what is genuine in religion and not hypocritical. The Wife of Bath represents not only bluntness and honesty but also the rising independence of women and the refutation of gender double standards, even at this early point in modern history.
It's not only a cross-section of jobs and societal roles but a cross-section of ideas and philosophies of life. It is a microcosm of England, Europe, and the world, but also a microcosm of the past, present, and future. Only a few decades earlier, Europe had gone through an apocalyptic upheaval when bubonic plague wiped out nearly half the population. In the aftermath and recovery from this catastrophe, there was almost a paradoxical sense of buoyancy and optimism implicit in people's thinking. It is a microcosm of this universal feeling, as well, that one senses in Chaucer's Prologue and in the Tales altogether.