Golden Rule

The books are merely guides; the GM makes the rules. If the GM says your spell didn't work or your car didn't start, accept it. Because if you don't he can make your character's penis fall off. If you don't like it, how about you GM next week? This is sometimes confused with, or even cited as Rule Zero, which is the statement of "The rules are guidelines, have fun.". Obviously, That Guy isn't a big fan of either.

Definition

The Golden Rule: "The GM makes the rules; don't argue with the GM"

In Depth

The rule boils down to this: the GM is God. It's the GM's job to make the story, and to make a party of Murderhobos try to get along, and maintain some sort of context for the campaign as a whole. As such, he's empowered to take certain creative liberties if the players are doing something ridiculous but within the confines of the rules, or if the campaign has gone tits up somewhere along the line, or to call shenanigans on That Guy when his antics are getting too disruptive. Part of the Golden Rule is that the GM is given the chore/pleasure to play mediator between player, characters, and the story as it unfolds.

The books, as said above, are guides, and the GM ultimately has the final say regarding your characters and actions. It won't matter if your character hit the BBEG with a called shot to the flashing weak point using your vorpal pistol of salsafying with a natural 20. If he says Crystal Dragon Jesus shrugs it off, then he does.

In addition, the GM does hold a Social Contract (also known as Rule 0.5) with the players not to abuse or fuck with his players in any dickwad manner. If he uses a house rule, then he needs to be sure to let them know. This goes for any additional rulings, but he should have a fairly rational reason for it. It's acceptable to say, not let them roll up a Cleric because the party comes from a secular society, but not ok to say you can't roll up a Techpriest because he thinks they're fucking weird.

Now all of that said, the Golden Rule isn't a bad thing. If you have a good or reasonable GM, the Golden Rule can be make for some great times.

Citations

Many games won't come out and say this exactly, trying to cover The golden Rule and Rule Zero both under something vague like "These rules are just a guideline and not meant to get in the way of fun", but some games do put the golden rule in explicitly.

• Paranoia Troubleshooters (25th anniversary edition), Page 40 has perhaps the greatest description of the Golden Rule out there:

GM Rule #1. You are IN CHARGE. You are ALWAYS RIGHT.

We give you these rules as guidance. Use them when you do not know what you’d like to have happen in the game. When you do know, ignore them. We have tried to make the rules as helpful and powerful as we can, but if you don’t like a rule, the rule is wrong. Good rules help a lot but bad rules were made to be broken, tortured, lobotomised and summarily executed. Dice are handy for giving players the illusion they control their destiny. This is valuable but roll your dice out of the players’ sight, behind a screen. If a die roll gives you a result you don’t like, the die is wrong. Change the result to the number you want. You can dock the die credits or beat it up, though in our experience this has little effect.

Emphasis not added. That's how it is in the book.