The age of Chaucer, which encompassed the second half of the fourteenth century, was a time of major change in Chaucer's England and in Europe as a whole. Chaucer's famous work The Canterbury Tales is evidence of one of these changes, namely writing in the vernacular, which is often associated with modern literature.
But larger forces were changing as well, many of which may be characterized as essentially modern. European kingdoms, notably England and France, began to consolidate into what might be called nation-states, with strong bureaucracies and the ability to collect taxes and marshal large military forces. The period also saw the decline of serfdom, especially in England, where severe losses to the workforce due to the Black Death enable agricultural laborers to negotiate wages.
Finally, the period saw increased criticism of the Catholic Church, due to corruption and especially the absurdity of the Babylonian Captivity and the Western Schism. One tangible critique of the Church emerged in Chaucer's England itself, where John Wyclif translated the Bible into English, a heresy to the medieval Church. A less tangible manifestation was the emergence of a secular spirit, one form of which was humanism.
In summation, while Chaucer's age was still decidedly medieval, it also saw many of the changes that historians associate with early modern Europe.