In the Friar's Tale, Chaucer exposes the hypocrisy of the clergy as he has the Friar interact with the Summoner. For, the Friar relates a tale about a summoner, then later the Summoner tells a tale about a friar.

The Friar's story is in the form of a fabliau, which is a short, cynical narrative composed in verse. In this tale, the summoner is ridiculed because he is tricked so easily by the devil after he has boasted of his exploits.

The theme of the tale is that the summoner, who has no limitations on what he will do to people, is worse than the devil since the devil can only take the summoner when the curse put upon him is genuine, as it is from the old woman.

Critics also regard this tale as an exemplum, a well-told story of egregious behavior with a moral ending (according to eNotes' study guide) as the summoner is punished by being taken by the devil. Thus, the moral of the story is that in being so consumed with trying to trick others, the summoner leaves himself open to be tricked by the devil. That is, his deceptions have caused his own downfall.

The summoner boasts of how he earns his livelihood by extortion and "sheer deceit." He works for an archdeacon: 

No rascal craftier in this British land,
For he'd created a skillful net of spies,
Who watch for him as they roamed (procurement eyes)....
...had plenty of pimps and panders ready
At hand; like hawks that swoop from out of view
They hunted secrets and told him all they knew....

The Friar goes as far as to say that Summoners are worse than the devil because the Devil has limits with his job (he knew the man did not mean the curse against his horse and wagon), while the Summoner had no limits, and he is destroyed by the Devil as a result (the woman's curse was genuine).

The Friar's tale is about a summoner, or a person who exposes sinners, who indulges his greed by extorting money from those he catches, letting them go if they will pay him what he asks. 

The theme of the tale is the close interconnection between greed and the devil.  The moral is that those who engage in extortion and greed will soon find themselves in hell.