An allegory is a type of tale in which the figures, characters or actions of the plot are meant to be symbols, which convey generalizations about humanity. In this example, "The Wife of Bath's Tale" is an example of an allegory.
The tale describes a knight who takes advantage of a maiden, and takes her virginity. As punishment, he is required to find what women desire the most. He eventually finds an old, ugly woman who will tell him the secret, but in exchange of marrying her. The secret is that women's greatest desire is to have power over their husband's.
This story is an allegory for the real relationships of people and more importantly, for the concept of sovereignty. Initially, the knight holds all the power, as he is able to abuse the maiden. However, as the story progresses, the knight loses his power against women, first because of the Queen, then because of the old woman, and finally because of the beautiful woman the old woman turns into. This is meant to symbolize the fight for autonomy that takes place within couples. Moreover, it is meant to show the subtle ways in which women might gain more power within a marriage.