Symbolism, Imagery, Allegory
It's probable that the pilgrims' journey from London to Canterbury represents another journey that was very important to a medieval person: the journey from Earth to Heaven. As the journey begins, we have a sinful group of pilgrims, many of whom are hiding various vices and dirty secrets. Their pilgrimage is meant to be a journey of repentance, so that by the time they reach Canterbury, they will be fully cleansed of these sins. Thus, in this allegory, the tavern represents the sinful life on Earth, while Canterbury represents the sin-free life in heaven all people are trying to reach.