Fourth Wall

The Fourth Wall is commonly the only thing separating you, the reader, from me, the text.

That's right. I know you're there!

Without the Fourth Wall, characters would notice you watching them, there might be references to the medium they exist in, and there might even be reader-character interaction. Most of the time, the Fourth Wall is carefully maintained so the reader stays immersed in the story and doesn't get distracted by meta stuff like that.

If the reader's immersion is disrupted such that they notice the fact that they are reading a story, this is referred to as breaking the Fourth Wall. Deliberately breaking the Fourth Wall is common in humorfic or in stories written to give the impression of the narrator telling the story to an audience (in which case the story can range from humorous to quite serious).

The Fourth Wall and the PPC
A hazard of writing fiction about fiction is that the Fourth Wall can get pretty thin. Some PPC authors gleefully smash right through it for comedy.

PPC agents, too, are keenly aware that the difference between fiction and reality is a matter of perspective at most. This has resulted in many agents knowing that they themselves are fictional characters (from a certain point of view) and have authors. These agents break the Fourth Wall just by existing. Some pretend not to notice, but others wink at the Fourth Wall for fun or shout at their authors when they get frustrated.