The fourth wall is the only thing keeping the characters from realising they're fictional and chatting with the audience. The term comes from theatre, in which the set had three walls, and the fourth was an imaginary, invisible one positioned between the actors and audience. The Way Of The Metagamer has absolutely no fourth wall, as from the very first comic the characters demonstrate acute awareness of their fictional status.
Although the fourth wall is broken in the first comic, as the comic progresses the remaining shards of the wall are jumped on repeatedly until they're ground into a fine powder, which is then baked into a cake. The characters regularly read each others' speech bubbles, look at other panels on the page, and go back through the archives. Plot holes are used as transportation, panels are pulled down off the comic, and sections of panels undock to become transportation modules.