The fourth wall is a performance convention in which an invisible, imagined wall separates actors from the audience. While the audience can see through this "wall", the convention assumes, the actors act as if they cannot. wikipedia:
I grew up in the middle of nowhere, saw my first concert in 1977 (Led Zeppelin), and wanted to work on this kind of stuff and so I went to school for technical theatre in 1981 because it was the closest thing I could find in those days to concert touring. Working backstage on theatre productions was really fun, and that took me into a long run of working on all kinds of shows, into grad school in 1987 and into a bunch of theatre sound design. It was all fine but I think sometime around 1998, when I did my last theatrical sound design, I finally realized: I don’t like the fourth wall.
I mean I’ve seen or worked on probably literally a thousand shows, and I knew this all along, but somewhere in there I realized that the shows I most enjoy--music, variety, comedy, circus, vaudeville, magic and even educational lectures--all acknowledge that the audience is actually there. In the same way that I don’t like realism onstage (when film does it much better) it just seems very weird to me to have the performers pretend the audience isn’t there. Of course, the boundary is always mutable, and some decorum must be maintained. And I especially like it when, in fact, the performers have a give and take with the audience. But isn’t that why we’re all there? We are there for the performers and the performers are there to be with us. Why put up a barrier?