The tale tells us that women in the Middle Ages were expected to occupy a position of subordination in relation to their menfolk yet still retained a certain measure of control over them in domestic affairs.

As the errant knight of the story discovers, what women want most of all is sovereignty over their husbands. Yet such sovereignty, such control, paradoxically takes place within a social structure in which women lack political or economic power. What we see here is a clear distinction between the domestic realm—the realm of hearth and home—and the public world, the world of business and politics. It is in the former of these two spheres that women exercise control.

The crone who turns into a beautiful young woman after marrying the knight will exercise sovereignty over her husband behind closed doors, far away from prying eyes. In doing so, she might very well act like Guinevere in Le Morte d'Arthur and get involved in high politics behind the scenes. But her public role will remain that of a loyal and dutiful wife, faithful to her husband and always displaying absolute fidelity to the roles to which she's been assigned by society: those of wife and mother.

The other answers to this question do a great job of foregrounding the historical context that the Wife of Bath's Prologue and Tale respond to. With that context in mind, it's interesting to think about the Wife of Bath's Tale not only illustrating what women's life was like in England during the Middle Ages, but also what life might be like if women were given more power. Indeed, we can read this section of The Canterbury Tales as a kind of feminist revisionary text, in which the Wife of Bath creates a poetic space that reclaims a role of power for women. Though she highlights the fact that women are obligated to marry and are routinely made the object of their marriages (it's particularly telling that she is primarily known as the Wife of Bath, rather than Alys or Alyson, as if her only identity revolves around her marital status), the Wife of Bath also creates a poetic space in which a woman fashions a marriage of equality, wherein both the husband and the wife have power. It's an interesting way of reading the poem and, if it doesn't tell us anything new about the role of women in the Middle Ages, it at least foregrounds the Wife of Bath's Tale as a feminist text, seeking to claim a revisionary role of power and influence for women.

The Wife of Bath's tale illustrates that women had less power and respect in marriage than they would have liked and that both men and women were largely clueless about women's desire for more authority. It also shows that for women in the Middle Ages the expected role in life was as a wife.

In the tale, a knight rapes a woman, and King Arthur turns him over to Queen Guinevere for justice. Guinevere tells the knight she will spare his life if he can tell her what it is women most desire. She gives him a year and a day to find out. You would think that would be plenty of time, but the knight himself doesn't know the answer and neither do the women he queries, as they all give him different responses that don't get to the core desire.

The old woman he is forced to marry in return for the needed information gives him the right answer: women most desire to rule over their husbands. He then finds out the great rewards of offering a wife authority and autonomy when he tells his new wife that she can decide whether he wants to be ugly by day or by night. When he gives her the power to decide, she makes herself beautiful by both day and night.

The tale shows that both men and women benefit when a woman is given power in a marriage.

Chaucer is commenting here about the narrow rules and confines women had to live by within a male-dominated society. In the Middle Ages, it was believed that men were born with the "higher" human nature. Men were "blessed" with rationality: they had all the brain power. Women, on the other hand, were viewed as ruled by their emotions, consumed by materialism and sexually insatiable.

Women's positions in life were further confined by the medieval church, which insisted women needed to be "tamed," yet promoted the need to be "fruitful and mulitply." The wife of Bath laments in lines 75-78,

For hadde God comanded maidenhede,
Than hadde he dampned wedding with the deede;
And certes, if there were no seed ysowe,
Virginitee, thanne wherof sholde it grow?

Another conflict women in the Middle Ages faced in both the Church and in society was in the need for women to be married to survive. Women could not be property owners, nor maintain independent income apart from a man. If one was unmarried, their choices were few: nunneries or becoming a burden to a family member. Thus, while the wife of Bath may seem bawdy and boastful at times with her tales of five husbands, society and the church in reality left women little choice.