Rule Zero is the commonly unspoken but seemingly well understood rule that, in the simplest of terms, “what the GM says...goes”.
A more complex version of this rule states that the GM is free to override any rule that he or she does not like.
Speaking as both a GM and a player, I think that this rule is, generally speaking, bullshit. It is a rule heavily leaned on by GMs who don’t know the rules of the game or poor GMs who try to micro-manage the characters in their campaign.
To be clear - The GM is the god of the game and what they say is law. Absolutely. Without this, you would have non-stop arguments or a bunch of people just sitting around a table angrily glaring at each other. For hours.
However, the rules need to be followed. If the rules are unclear or nonexistent, then that’s a problem with the rules.
The rules of a TTRPG are a contract between the GM and the players as to how the world works.
For example, in “real life”, if I jump out of a window on to a sidewise 30 feet below me, I would likely injure myself greatly. Perhaps your personal experience would be different and you would land, perform a forward summersault and leap to your feat unscathed. I don’t know. But, this is the real world I’m talking about.
In classic D&D, if a character falls 30 feet on to a solid stone floor, they sustain 3d6 damage.
* Armor does not have any effect. A character wearing robes would take the same damage as a character in full plate armor.
* A character who had 20 hit points would survive the fall, no matter what, since the highest amount of damage is 18.
In Rolemaster, a character who falls 30 feet takes a +30 Fall/Crush attack.
* Armor affects the amount of damage sustained.
* Because all attacks are open-ended, there is a possibility that the attack could do an incredible amount of damage and the character could sustain a critical hit that breaks every bone in their body. Probably not, but there is a chance. Likewise, there is a chance, however unlikely, that no damage at all is sustained.
In GURPS, a character who falls 30 feet on to a solid stone floor takes 4d damage, assuming they are of average strength.
* Armor does help absorb this damage, through characters still take about 20% of the damage from the fall.
* A character with an average strength has only 10 hit points usually, and rolling 4d6 would almost certainly cause serious damage, likely rendering them unconscious.
When a classic D&D Fighter with 20 hit points sees a cliff with a 30 foot drop to the road below, but is being chased by a horde of goblins, or a giant boulder, or Demogorgon, he can make a choice. He can decide to jump off the side, knowing that he will survive.
At that moment, the GM tells a story about how his friend Merle fell out of a tree last year and only fell 7 feet on to his lawn, but he broke both legs and ruptured his spleen and was in the hospital for 6 weeks. So, the GM decides at this moment that falls should be more realistic and decides that characters sustain 1d6 damage for every five foot fallen, instead of every ten. Another very unhelpful player mentions how objects falling accelerate, so the amount of damage might double every X number of feet fallen, up to a maximum velocity of 200 feet a second. Based on this, the GM decides that it should do 1d6 damage for the first 5 feet fallen, then doubled to 2d6 for 6 to 10 feet, then doubled to 4d6 for 11 to 15 feet, then doubled to 8d6 for 16 to 20 feet, 16d6 for 21 to 25, and 32d6 for 26 to 30 feet.
The GM rolls 32d6 and gets a low-average of 94 points of damage, announcing that the fighter has been reduced to -74 hp and is annihilated.