Without the "General Prologue to the Canterbury Tales," none of the other stories which comprise Geoffrey Chaucer's The Canterbury Tales would make as much sense or, frankly, have as much purpose. The "Prologue" provides the context for understanding the Tales.
The purpose of the Prologue is twofold: to introduce the characters who are making this pilgrimage and to set the framework for the stories to follow.
When April with his showers sweet with fruit
The drought of March has pierced unto the root
And bathed each vein with liquor that has power...
Then do folk long to go on pilgrimage....
We learn that these characters are all making a holy pilgrimage to the church at Canterbury, a popular religious destination after Thomas Beckett, a priest, was murdered and proclaimed a saint. After we meet them all, we are privy to a proposition made to the pilgrims by the innkeeper at the inn where they are staying for the night. Each of them will tell two stories on the way to Canterbury and two stories on the way back; whoever tells the
[t]ales of best sense, in most amusing mode,
will receive a free meal at the end of the journey, paid for by the rest of the travelers. This is the reason for the tales they all tell, The Canterbury Tales. Each of them is competing to tell the best tale (defined, of course, by their own standards) in order to win the prize. That is the premise behind all the storytelling.
Chaucer introduces us to the pilgrims, and they are indeed an interesting lot. They range from the noble and humble knight to the despicable Pardoner who dupes poor, godly parsons into buying bones which he falsely claims are holy relics. Many of the worst moral offenders are the clerics and nuns, concerned more about their own pleasures than about either God or His people. The Friar is one of those:
In towns he knew the taverns, every one,
And every good host and each barmaid too-
Better than begging lepers, these he knew.
For unto no such solid man as he
Accorded it, as far as he could see,
To have sick lepers for acquaintances.
There is no honest advantageousness
In dealing with such poverty-stricken curs;
It's with the rich and with big victuallers.
Despite their flaws, Chaucer introduces them to us more as sinful scoundrels than as people to be despised. As we meet them, we are appalled at their outrageousness but not overly incensed by their excessive displays of humanity.
Unlike the famous tales that follow it, the "Prologue" does not attempt to deliver a particular moral message or statement. Instead, the colorful list of characters reminds of us of the propensity of men (and women) to sin--and enjoy it. He does show us virtuous characters, perhaps as a foil to the more sinful ones. For example, in contrast to the Prioress, who tries to emulate the manners of the worldly women at court, is the Parson, a humble man who will even pay his parishioners' tithes out of his own pocket if they are unable to do so.
A country parson, poor, I warrant you;
But rich he was in holy thought and work.
He was a learned man also, a clerk,
Who Christ's own gospel truly sought to preach;
Devoutly his parishioners would he teach.
This cast of characters is representative of English society in the fourteenth century, both in its profound goodness and its extreme self-centeredness.
If we think of "theme" as being a kind of moral, we have to read The Canterbury Tales to get that; in the "Prologue" we get another kind of theme: the subject matter of things to come.