The Fourth Wall is a metaphorical, metafictional barrier that separates the readers from the characters, or the fictional world from the Real World.
The lifeless body of the Mechanical Author is embedded into the Fourth Wall after he was defeated by the Ultimate Savior for his attempt to break through the wall and kill all of the LNH Writers behind it.
There are also First through Third Walls, which seem to be part of the makeup of the Looniverses, and are essential to its stability. They separate the LNH from "lower" fictional worlds, such as that of the Teenage Disco Vampire Barbershop Quartet Net.Force. It's said if these walls were broken, the Looniverse would be sucked into the Real World, or at least Real World Minus One.
Then there's the Fifth Wall...