"Don't be a dick."
It essentially means players should treat each friendly and with respect.
If someone intentionally does something inappropriate, or acts maliciously towards another player or players, he's breaking Rule 0.
Particular behavior breaking the rule may differ according to a game you are playing. For example, some RPGs are designed as competitive games, while others are cooperative. Behaviour common in the first type could be sometimes considered bad (breaking the Rule 0) in the latter.
* other than "I wanted your loot / I wanted to earn XP by killing you / I did it for the lulz" etc...
The Game/Dungeon Master has the right to veto anything any player says, he has the right to change any rule or make up his own, he need not explain why he choses to do these things. If players complain the GM may choose any of the following to do to the player; slap, call a dumbass, restrict snackage privileges and/or threaten injury to ingame character(be it through loss of xp, health, items or gold)
DM: Hells no! I'm gonna have to Rule 0 that one
DM: I swear I will cut you off of the mountain dew boy
Player: I'm sorry master.
GM: I'm invoking Rule 0. No.
The unwritten rule in tabletop role-playing games (such as Dungeons & Dragons) which grants the game master the right to suspend or override the published game rules whenever s/he deems necessary.
Similar to a house rule; however, rule zero may be invoked unilaterally, at any time and does not have to be agreed upon in advance by the players.
When abused, can lead to an inconsistent, confusing, or frustrating experience for the players.
GM: The wall is magically warded against climbing. You fail.
Bob: But a 20 is an automatic success!
GM: Rule 0. You fail.
rule 0 person: OMGZZZ! (insert name here) loveezzz buttter. oh i miss him so much. i love my babyyyyyyy.