Most good stories start with a fundamental list of ingredients: the initial situation, conflict, complication, climax, suspense, denouement, and conclusion. Great writers sometimes shake up the recipe and add some spice.
Initial Situation
A group of pilgrims have gathered in a tavern just outside of London in preparation for their journey to Canterbury. There, our narrator Chaucer meets them and becomes one of their fellowship.
This is the situation that leads to all the rest of the action. Were the pilgrims not gathered here, there would be no group pilgrimage, no need for entertainment and hence tale-telling, and no Canterbury Tales. Were Chaucer not to happen upon them at the tavern, there would be no occasion for him to narrate the Tales.
Conflict
The Host proposes a tale-telling game to entertain the pilgrims on their way to Canterbury. The goal? To "be merry," or, have fun, on what could otherwise be a boring road trip. Also, to win the game (and the free dinner at the end) by telling the best tale.
This very obviously creates conflict between the pilgrims because each one wants to win the contest. It also creates a conflict between the Host's goals of an orderly and fun contest, and the natural proclivities of the pilgrims. This is the conflict that will turn out to be most important in the action that follows.
Complication
The Miller gets drunk and decides he wants to go after the Knight. The Reeve takes offense at the Miller's tale. The Friar and the Summoner don't like one another. A mysterious stranger appears in the distance. These are just a few examples. Really, anything that threatens the orderly telling of tales could qualify as a complication.
All of these situations – the Miller's drunken obstinance, the Reeve's reaction to the Miller's tale, the Friar and the Summoner's dislike of one another, the appearance of a mysterious stranger in the distance – complicate the Host's plan to have the tale-tellers tell stories in an orderly fashion, and to have fun, because they have the potential to create interruptions and fights.
Climax
The first when the drunken Miller interrupts the order of tale-telling the Host proposes. The next, when the Reeve takes offense at the Miller's tale and takes the opportunity to tell a mean story about millers. Same thing happens with the Friar and the Summoner. Oh, and the Canon and Canon's Yeoman also interrupt the pilgrims mid-way through their journey.
The things that seemed likely to happen in the complication stage actually happen.
Suspense
Again, many possibilities here: the Miller threatens to leave the pilgrimage if he doesn't get to follow the Knight. The Reeve asks to be allowed only one thing: to tell his tale next. The Host asks the Friar to quit insulting the Summoner and tell his tale, already. The Summoner tells a crude story about Friars in the Devil's anus. The Host asks the Canon's Yeoman if his master is capable of telling a good tale.
All of these moments represent times when things are about to get very, very bad for the Host's stated goals of having fun and telling tales. If the Miller were to leave in anger, he would definitely not be having fun and the harmony of the group would be further disrupted. The same deal would happen if the Host were to refuse to let the Reeve tell his tale. And what if the Friar took so much offense at the Summoner's story, he decked the Summoner, rather than allowing him to tell his tale? You get the idea.
Denouement
The Host allows the Miller to tell his tale. The same happens with the Reeve. The Friar quits insulting the Summoner and tells his tale. The Friar doesn't deck the Summoner, who goes on to tell his tale. The Canon's Yeoman turns out to be a nice guy who tells an interesting tale about alchemy.
Here, the conflicts that were getting in the way of the tale-telling and fun are resolved, if not entirely to everyone's satisfaction, then at least enough that the tale-telling can continue.
Conclusion
There really is no conclusion, since we never see the Pilgrims conclude their journey, nor do we learn who wins the tale-telling contest.
The end of the journey and the announcement of a winner to the contest would conclude the action that's set in motion in the conflict stage. We would find out not only who wins the contest, but whether the Host's plan for everybody to tell tales and have fun was successful.