What Is Chaucer's Prologue To The "Canterbury Tales" About?

The General  Prologue to the Canterbury Tales introduces us to the characters - the pilgrims who will tell the Tales. It's  what is called a 'framing narrative' -  a story to give us a background to the main story. The 30 or so characters are on a pilgrimage to visit a holy place in Canterbury, and before starting they spend the night at a London inn. The Host of the Inn decides to go along with them, and suggests that to pass the time on the journey, they all tell stories. They agree, and the stories they tell are in fact the Canterbury Tales. During the Prologue, Chaucer also gives us a sketch of each pilgrim so that we know their characters -  you can find some examples here.

Preserve their stories for the future