"The play's the thing wherein I'll catch the conscience of the King."
When exclaiming "The play's the thing!" we're seldom asked the embarrassing question of what "thing" we mean. Prince Hamlet, however, has something specific in mind. To secure proof that his uncle, King Claudius, murdered Hamlet's father, the former king. In order to do this he includes a few telling lines into a play about regicide knowing that his uncle will be watching at court. Hamlet then waits to see if Claudius will flinch. If Hamlet's plan works, he'll be convinced of the king's guilt and will feel better about taking revenge on his uncle.
It is at the heart of every play that though they are works of fiction, they mirror real life in such a way that the audience says, "I've been there. I know that. I recognize him or her", and somewhere in the middle of a laugh or tear comes a deeper understanding of the human condition. Great plays get recognized as great simply because they contain something universal that at one time or another the audience feels is a part of their life.
One of the best and most often produced is "Our Town". At some point we have seen it in a high school auditorium, on television, in any number of remakes. We have been dragged into playing one of the characters , helped with the scene changes of the minimal staging. Even people who hate theater probably have some knowledge of this Thornton Wilder play that even with being about a small town in a midwestern state, at a time removed from our hectic lives still somehow manages to touch us with moments of understanding.
As you progress through the lives of the characters eventually you come to the realization as spoken at the end "My, wasn't life awful--and wonderful." The ultimate message is that we never really notice the importance of our lives while we live them but a great play can make us sit up and pay attention.