Chaucer is a poet of transition between a conversational colloquial style and a lofty poetic style.
As Dante and Petrarch, two poets Chaucer cites throughout the Canterbury Tales, exalted the Italian language through the refinement of their native Tuscan vernacular, Chaucer enriches English. He does this first by choosing to engage with English when many of his contemporaries preferred to write in Latin, and second by borrowing not just plots but words from other languages, particularly French.
Beyond this, he is able to navigate the idiomatic language of his pilgrim characters without debasing his high form. The language is direct, emphatic and immediate, while still triumphing not just by evoking the literary muses, but also in his high style particularly in the opening lines of the Prologue.