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

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

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

When an Аниме person knows there in an Аниме или somtin like that

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

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

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

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