What agreement did they make with each other?

The pilgrims decide to tell stories to pass their time on the way to Canterbury. The Host of the Tabard Inn sets the rules for the tales. Each of the pilgrims will tell two stories on the way to Canterbury, and two stories on the return trip. The Host will decide whose tale is best for meaningfulness and for fun. They decide to draw lots to see who will tell the first tale, and the Knight receives the honor.