Note/Disclaimer: This info is taken directly from VS Battles Wiki
The Fourth Wall is a literary term that originates from stage plays. Typically, a stage would be rectangular, so there would be 3 walls, one in the back, and 2 on the sides. The "fourth wall" would be the edge of the stage that faces the audience. Obviously, there was no actual wall there. The term Fourth Wall refers to the barrier between the audience of a work of fiction and the fictional universe itself. In most works of fiction, the fourth wall is intact, that is, the characters do not acknowledge the existence of an audience or that they themselves are part of a fictional work. However, some fictions employ the literary technique of Breaking the Fourth Wall. This includes talking to the audience, affecting the "real world", rewriting speech bubbles, acknowledging that they are part of a fictional world, and even leaving the fictional work itself and entering the "real world" (represented, obviously, by a fictionalized version of the real world). It is one of the most versatile abilities out there.