As the nun's priest (or nonne preestes) sums it up in the end:

Lo, swich it is for to be recchelees,
And necligent, and truste on flaterye!

The meaning behind the tale is a admonishment not to fall prey to vain flattery in the way that Chauntecleer fell prey to the fox's flattery. The contrasting example that Chaucer builds into the tale to show the correct manner of thought and conduct and the opposite of flattery is the debate between Pertelote and Chauntecleer about the nature of dreams. Pertelote is forthright in expressing her opinion and is straightforward in her pronouncements about Chauntecleer. In other words, she is genuine and does not flatter.

Click on this link for a summary and analysis of the story:

http://www.enotes.com/canterbury/11-nuns-priests-tale-summary-analysis