What is an example for catharsis?

Have you ever seen a good film with a group of friends and, after the show, felt as you could not go home and sleep unless you talked it over and thoroughly dissected the subject among the group? This is "catharsis"

Literally "catharsis" means “to purify”; from the Greek verb κάθαιρo, pronounced catairo.

According to Aristotle it is the good feeling that any spectator should feel after witnessing a dramatic event or a tragic representation. You have seen it happen. You have had the fortune of not being personally part of it. You can now step back, consider the facts and draw the ethical conclusions.

As you have done so you feel better, and will have no problem in dreaming sweet dreams.