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

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

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

When an Anime person knows there in an Anime o somtin like that

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

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

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

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