"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.
(Jak 3:)
Monk: This isn't a game!
-Jak and Daxter look toward the player with confused expressions-
Your first piss in the pub, usually after 2 hours of drinking. After breaking the seal of your bladder, repeat visits to the toilet will be required every 10 or 15 minutes for the rest of the night.
'I broke the seal in the Flapper and Firkin bogs, after that there was no stopping me"
Nickname given to bales of marijuana thrown overboard or out of airplanes. Origin is reportedly from members of the US Coast Guard in the area of Southern Florida.