hedwards When you are watching a stage play from the audience and the play is set in, say, an apartment, you'll see the back wall of the apartment, the left wall and the right wall. Those are three walls. The missing fourth wall is the one missing so you can watch the play but the actors must pretend the fourth wall is there and there is no audience but, should the actor step into that space and address the audience, that would be breaking the fourth wall and is usually not done because it breaks the contract with the audience--the illusion of reality.
I mis-typed "the third wall" earlier and there is such a thing when an actor removes himself from the business on stage and, hence, the play itself but not so far as to interact with the audience. He may go off to the side and give a speech that doesn't involve the other action onstage.
But these are deeper technicalities I would let him explain but we're not speaking right now. He asked me, "To be or not to be?" and I replied, "What kind of question is that?"