What's "The Fourth Wall"? :-3

The Fourth 벽 is the 벽 that separates reality from fiction. It's the 벽 between, say, a cartoon and the real world. If a character in a show talks 또는 looks directly at the audience, even though they are in a fictional world, that is called, 'fourth 벽 breakage.'
An example from Hayate no Gotoku:

It when an 아니메 character realizes what they are here is a example of a fourth wall

When an 아니메 person knows there in an 아니메 또는 somtin like that

당신 should be asking this on the 영화 팬 club 또는 something. But anyway...
If 당신 won't just look it up like 당신 should, (Wikipedia has it) the 4th 벽 is a metaphorical "wall" referring to, in a play, movie 또는 any such thing, the unseen and uncounted direction in the scene which points to the audience. As this direction cannot exist in the theatre, it is blocked out as an impenetrable wall. Or, rather, impenetrable in most cases. Therefore, when the fictional character breaks the first wall, it's when he/she is talking to the audience. As this would shatter a solid enough reality, it is, as 당신 probably already know, used mostly in the less mature works of theatre such as children's cartoons.

People have already answered this 질문 but I just wanted to add on my own thoughts. Just because an 아니메 character is facing the camera directly doesn't necessarily mean they are conscious of an audience being there. Like when an 아니메 character does a behind-the-shoulder twist to do an evil laugh that doesn't mean they are directing it at the audience it just means they're being sneaky, cheering to themselves silently, etc. To me, that isn't breaking the fourth wall.

Also, a narration of thoughts is not to be confused with breaking the fourth wall. If they were one in the same pretty much every 아니메 would have broken the wall. We're just getting to listen to their thoughts.

Advertisements break this 벽 constantly with things like "Ask 당신 doctor before trying...." and making eye contact with the audience.

4th 벽 breaking starts at 5:12. I swear those two are pros at it!