What is the "fourth wall" concept in theater and cinema production?

The fourth wall is a conceptual barrier between those presenting some kind of a communication and those receiving it. The term originated in the theater, where it refers to the imaginary wall at the front of the stage separating the audience from the performers.

Explanation:

The presence of the fourth wall is an established convention of modern realistic theatre, which has led some artists to draw direct attention to it for dramatic or comic effect when a boundary is "broken", when an actor or character addresses the audience directly.

The fourth wall is an imaginary wall between the audience and the actors, when the fourth wall is "broken" the actors interact with the audience.