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

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

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

When an animê person knows there in an animê or somtin like that

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

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

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

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