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

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

It when an 日本动漫 character realizes what they are here is a example of a fourth wall

When an 日本动漫 person knows there in an 日本动漫 或者 somtin like that

你 should be asking this on the 电影院 粉丝 club 或者 something. But anyway...
If 你 won't just look it up like 你 should, (Wikipedia has it) the 4th 墙 is a metaphorical "wall" referring to, in a play, movie 或者 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 你 probably already know, used mostly in the less mature works of theatre such as children's cartoons.

People have already answered this 问题 but I just wanted to add on my own thoughts. Just because an 日本动漫 character is facing the camera directly doesn't necessarily mean they are conscious of an audience being there. Like when an 日本动漫 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 日本动漫 would have broken the wall. We're just getting to listen to their thoughts.

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

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