Scholars are still debating what Aristotle meant when he introduced the idea of catharsis. However, he writes that:
Tragedy [...] is an imitation of an action that is serious, complete, and of a certain magnitude; in language embellished with each kind of artistic ornament, the several kinds being found in separate parts of the play; in the form of action, not of narrative; through pity and fear effecting the proper purgation of these emotions.
This "purg[ing]" he refers to at the very end of this statement is catharsis. Throughout a tragedy, which imitates action, the revelation of action ought to produce, among audience members, pity and fear for the protagonist (or the chief actor in the tragedy: the tragic hero). The eventual exposure of truth or whatever needs to occur in order for the protagonist to experience his inevitable ruin is what produces catharsis, or the purging of this emotion, for the audience. In other words, then, it is we, the audience, who experience the catharsis once the protagonist understands his position and the role he has played in bringing about his own ultimate ruin.