The message delivered in the Wife of Bath's tale is related to the quest embarked upon by the knight in the story. In order to avoid being put to death for raping a young maiden, the knight has to find the answer to the question “What do women most desire?”

On the face of it, it doesn't seem that difficult a question to answer. But as the knight soon discovers, it's not as easy as one might think. He talks to many different women on his travels, and each one gives him a completely different answer. It's only when he comes across an old woman that he finally gets an answer to his question, which is that what women most desire is sovereignty over their men.

In return for receiving the answer to his question, the knight must marry this hideous old hag, which fills him with dread. What's more, he's forced to make a choice between having a wife who is ugly and faithful or beautiful and inconstant. However, his wise decision to choose the former pays dividends, as the old crone is suddenly and magically transformed into a beautiful young woman—and a faithful one at that. The sovereignty that the old woman enjoys over her husband has clearly worked out very well for both of them.

The theme that is clearly displayed in "The Wife of Bath's Tale" from Chaucer's Canterbury Tales is that women desire to have the same sovereignty over men that men have over them or, simply, to be equal to men and that when this lesson is manifested, the resulting union is a happy one.

The knight receives the answer of equality from the old crone as he seeks the answer to the question of what women want most that was posed to him by the queen. However, once he answers correctly, he has to marry the old woman and is put to the test by her to see whether or not he has truly learned this truth. She asks if he would rather have an ugly faithful wife or a beautiful promiscuous wife. He answers wisely by letting her decide by saying: "In honor to us both, I don't care which;/ Whatever pleases you suffices me" (410-411). The knight passes the test, and the reader is told that as a result of his newfound wisdom he is blessed with a life of "perfect bliss."

The theme, or universal statement, that "The Wife of Bath's Tale" ultimately makes regards subverting gender norms that were typical of the Middle Ages. According to canon law, or the law of the Catholic Church and the prevailing law throughout the Middle Ages, marriage was exclusive and male-dominated. The Wife of Bath, however, portrays marriage as female-dominated and governed by lust as opposed to divine imitation:

Of tribulacion in mariage,
Of which I am expert in al myn age
This is to seyn, myself have been the whippe (III.179–181).

In describing herself as a "whippe," or whip, she portrays herself as a dominate taskmaster: one who controls an animal or slave. She places herself outside of the control of men. Additionally, the physicality of the whip suggests that marriage contains a certain carnal element hitherto unemphasized throughout Chaucer's time.

In fact, her very title as "the wife" is ironic for two reasons. First, she is not a definite wife of only one man; instead, she is an indefinite wife of many. Also, she starkly contradicts "typical" wife behavior.

Finally, of course, her story's conclusion asserts that what women want is power over their husbands; this assertion makes clear the Wife of Bath's objective to subvert traditional medieval gender roles by reversing canon-law power dynamics.

I hope this helped! For more information, please check out the eNotes guide to this great collection of stories, linked below.

The message that is delivered is the answer to the question "What do all women most desire?"  Through the Wife of Bath's very long prologue and following story, the reader learns that her answer to that question is "To have power and control over their husbands and lovers."  

Her prologue illustrates this concept because in it the Wife of Bath illustrates how she has had five husbands and tells the travelers exactly how she attempted to manipulate each husband into doing what she wanted.  At times she used fear; other times she used her sexuality.  

The Wife of Bath's tale follows a similar story arc.  A knight is sent out on a quest to find out what women most desire.  He comes back with the answer that an old hag tells him.  Women want control over their man.  The court accepts his answer and as payment the knight must forever be with the old hag.  He's not happy about it, but the hag then gives him a choice: she can turn beautiful but be unfaithful or she can be faithful and loving but ugly.  The knight tells her to pick which she would rather be, thus giving her exactly what she most desires -- the power to control her own decisions.  So she turns beautiful AND faithful.