Breaking the Fourth Wall

"The fourth wall" is an expression stemming from the world of theater. In most modern theater design, a room will consist of three physical walls, as well as a an imaginary fourth that serves to separate the world of the characters from that of the audience.

In fiction, "breaking the fourth wall" often means having a character become aware of their fictional nature. This can range from your character advising you to "Press X" in a tutorial all the way to Psycho Mantis reading your memory card and mentioning the other games you've been playing. However, the most direct violation of the fourth wall would be a character openly acknowledging they are in a video game or even directly speaking to you, the player, instead of to your character.

Whenever characters in movies/video games/cartoons/other interact with the audience in some way

"The fourth wall" is the wall that separates fictional worlds from reality, so breaking the fourth wall is basically saying that you know that there's another universe outside of the fictional one.
Deadpool does this alot

In a piece of fiction, eg. a comic, novel, or theatre, for a character to acknowledge the fact that they are fictional.

You have to spin jump then backflip in a corner at 4:20 and you will clip into the shadow realm then type in the chat debug/remove wall:4 then debug/leave and then boom its gone