The fourteenth century was a time of enormous upheaval in England and all of Europe, mainly due to the Black Death at about the mid-point of the age, which is sometimes thought to have wiped out nearly half the population. Unsurprisingly, the pandemic affected the common people, the peasants, disproportionately. The wealthy people, the nobles, who could remain relatively isolated in their large dwellings, were disproportionately unaffected because they had less exposure to others who did have the disease. But those who lived communally—the great mass of people—were dropping like flies.
It stands to reason that literature, like anything else, would be affected by the massive social upheaval caused by this cataclysm. Anglo-Saxon and Anglo-Norman writings had centered around myth and legend, often dealing with issues of loyalty to a sovereign. The fealty of Beowulf to King Hrothgar is at the core of the great Old English epic. In the Anglo-Norman period the story of Tristan and Isolde (or Iseut), for instance, appeared in various versions with its archetypal conflict between the two lovers and Tristan's loyalty to King Mark.
It's only a slight exaggeration that in the late fourteenth century, when Chaucer reached maturity, the emphasis upon myth, courtly love, and legendary portrayals of royal authority became passé. The Canterbury Tales is all about real people, a cross-section of society. The slightly earlier Piers Plowman, though its characters are allegorical, is concerned with issues of sin and repentance as they largely affect ordinary people. Despite the appearance of a king in the dream visions that make up the long poem, his presence is almost an incidental one. The very act of plowing with its centrality to the poem is a symbol of the agricultural work that the great majority of people at the time engaged in.
Chaucer, in his The Canterbury Tales, is not just concerned with showing a cross section of the pilgrims to a shrine, but with the ideas they believe in and wish to express, including subversive ones. The Wife of Bath is a feminist 400 years before the first actual feminist movement led by Mary Wollstonecraft and others during the late Enlightenment and the period of the French Revolution. There is, as well, an anti-authoritarian tone to much of Chaucer, ranging from the bawdiness of "The Miller's Tale" to the allegory of "The Pardoner's Tale" in which the (often unacknowledged) cupidity at the root of human behavior propels the action.
The manner in which Chaucer's characters speak is so natural and unaffected (in a way that comes through to us despite the difficulties of reading Middle English) that we seem already to be given a look at an egalitarian era, again, about four centuries in advance of the time when the fact of equality among people was explicitly recognized in the Zeitgeist of the eighteenth-century Age of Reason.
Finally, the most significant thing about the late 1300s is arguably that these works were written in English. Norman French no longer had to be the language of literature. But at the same time what we see is Chaucer's enshrining, so to speak, in the Tales and his other works this transformed English with so much of its vocabulary consisting of borrowings from the language of the Norman conquerors. This mélange of Germanic and Romance vocabulary is in some way symbolic of the English people having become united, with the old division between Saxons and Normans a thing of the past. And the specifically London-based form of English in which Chaucer wrote established the standard, from this point on, for both the literary and spoken language.