Satire is defined as the opposite of what is expected either in oral or written work. In the Franklin's case, he is revealed in the general Prologue as a wealthy man who likes to be well thought of as a host--generous and always ready to give freely of his food and wine. He is also, however, quick to temper, dominated by the "blood humor"--in Medieval days the four humors were considered the basis of health and an imbalance of any of them caused a behavior problem. He beats the cook if the sauce isn't just right. One gets the impression that he is quick to punish all faults in his employees who must not love the Franklin much, no matter how generous or "worthy" he is.
The story this important and wealthy man tells is one of honor and noble behavior, regardless of social rank. He tells of a knight who woos a lady and establishes marriage on the basis of equality. They wed and love each other very much. When he must leave for an extended time period, she is left watching the horizon and longing for his ship's appearance. A squire tricks her into promising herself to him if he can make the jagged rocks which threaten her husband's ship to disappear. He does, and when her husband returns, he says she must keep her promise. The squire releases her from her vow, embarrassed by his behavior.
The satire is that the Franklin does not act nobly in his daily life, but he preaches it.