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.