The denouement is where all the tension that has built up during the climax is released. The protagonist can finally rest. If the protagonist did not obtain their want (goal) in the climax, then they will achieve their want (or goal) here. The want/goal is what the protagonist has been pursuing through the entire story.
This is also where readers see that the protagonist has gone through a full transformation. They are better, stronger, or a new person entirely, whether for good or for bad. They have learned and grown over the course of the story, and they are now better than they were before (or perhaps worse, depending on the type of story you are telling).
Loose ends and subplots are wrapped up.
Answers to questions and/or secrets are exposed.
This is where you leave readers with a satisfying ending