Geoffrey Chaucer, as the so-called “father of English poetry,” lived and wrote with one foot in the Middle Ages (Medieval) and the other in the period just preceding the Renaissance. We can call him both a medievalist and a modern because of his subject matter and the tone he takes with his subjects.
He starts his story with the character of the Knight. The Knight is medieval England personified. He exemplifies the medieval knightly code, chivalry, flawlessly. Then Chaucer takes an interesting twist with the next character, the Squire. The Squire is the Knight's son. As such, he assists the Knight and will someday become a knight himself. But he is less traditional than the Knight—look at two of Chaucer's last lines about him: "So hotly did he love at night / That he slept no more than a nightingale."
The Squire is much more “modern” than the knight, and appears to be engaging in some pre-marital amorous activity. This is something that appeared more often later in British literature, but not in Chaucer's time. It looks like Chaucer is suggesting a shift in thinking from father to son, which would indicate a shift in thinking from the Medieval period to a more modern period.
To finish answering this question, we need to look at Chaucer's historical context for a moment.
In the Middle Ages, most poets and artists produced works that were commissioned by the royal court or some other rich person. The only way they could afford to produce literature and art for a living was through the financial support of a wealthy person, usually one with some political power. There was no printing press yet, so poets could not make a living by selling their work on their own. So, since they depended on the support of the established class, they had to be fairly respectful toward that class in their work.
But with Chaucer we start to see the advent of a satirical, sometimes biting tone that later became a staple in British literature.
A look at several of his characters bears this out. The Friar, Pardoner, and Monk are all church officials. However, Chaucer is ruthless in his attack on their hypocrisy. They claim the advantages of working for the church, but use their positions to enrich themselves and engage in sinful pleasures. Little of what came before Chaucer shows this willingness to openly criticize the behavior of religious personages. Keep in mind that at this point in history, the church and the state were closely allied, so to attack the church was to also attack the state.
Later, in the Renaissance and the Enlightenment that followed Chaucer's time, this kind of satire became an important part of the British literary canon.