“Breaking through the fourth wall” means addressing Doonesbury readers directly. Imagine that a play takes place in a single room four walls within the context of a larger theater. Remove one of these walls. The audience of the theater sits in seats facing the remaining three walls. Normally, both the actors and the audience pretend that the fourth wall, the separating wall, remains intact. However, the playwright (in this case Trudeau) may have the actors (in this case Jeff ‘n’ Zipper) address the audience (in this case you and me) directly. This kind of action is known in theater as “breaking through the fourth wall”.