The "fourth wall" in a television show or play is the invisible wall which is missing so that the audience can look onto the set and see into the lives of the characters. Bill Prady wrote this about the fourth wall:
"...when it's used, the fourth wall blocks the audience. They watch monitors that have the live feed. Don't use the 4th wall much.
The fourth wall consists of generic wall set pieces which can be brought in to provide background for unusual camera angles. Examples include the view on the security camera of Leonard and Penny kissing in "Bad Fish Paradigm" and the kitchen wall seen in the background when we see from Penny's point of view while she's sitting in Sheldon's spot in "Cruciferous Vegetable Amplification".