- You may be looking for the audio named after this concept.
The fourth wall is a theatre term referring to the audience. This originates in the idea that there are three walls on a stage: one on the back, one to the left, and one to the right, as well as an imaginary fourth wall in front that contains the players within their play. To "break the fourth wall" means to show awareness of the audience or other things outside of it.
The fourth wall was famously broken in episode 7 of The Daleks' Master Plan, "The Feast of Steven", in which the First Doctor wishes the viewers a "happy Christmas": "Oh and incidentally, a happy Christmas to all of you at home." This is the only case in the series proper in which a character explicitly displays knowledge of being on TV; all other cases of fourth wall breaking involve the characters seeming to talk to or perform for the camera, but nothing that cannot be explained by another character or a mirror being in the position the camera is occupying or a character talking to them self, and nothing which displays knowledge of being a TV character.
In episode 2 of The Invasion of Time, the Fourth Doctor breaks the fourth wall by looking at the camera and quipping, "Even the sonic screwdriver won't get me out of this one." At the end of the same serial, he grins mischievously to the camera.
In Deep Breath, after the Half-Face Man has fallen from his "escape pod" and been skewered on a spike on the top of the Elizabeth Tower, the Twelfth Doctor looks directly into the camera as the question is posed: "Did the robot self-destruct or is the Doctor a murderer?"
The beginning of Before the Flood features a lengthy segment where the Twelfth Doctor talks directly to the audience and explains the "bootstrap paradox", the viewer to Google it. He uses an analogy of how he went back in time to meet his hero Ludwig van Beethoven, only to find out he would never write out the fifth symphony, so the Doctor copies it out based on his future knowledge, where Beethoven then publishes it. However this means the Doctor was inspired by Beethoven, who was inspired by the Doctor. He then leaves the viewer with the question "who wrote Beethoven's fifth?" before he takes out an electric guitar and plays the fifth symphony which then transitions to the Doctor Who theme.