"The rules" are spread across a number of locations. This post is an attempt to bring the basics of what this Stack expects of questions together into one place. Before I begin, please let me underscore something that gets ignored a lot on this Stack. From the Help Center we read:
Some subjective questions are allowed, but “subjective” does not mean “anything goes”. All subjective questions are expected to be constructive. What does that mean? Constructive subjective questions: ... are more than just mindless social fun.
We're all here to have fun, but asking questions because you're bored, or are "having fun," or are not really trying to ask questions either to further your own worldbuilding efforts or to further the "cause of worldbuilding,"1 is not welcome. That's a bit harsh, but the rule serves a purpose. It keeps the Stack from becoming cluttered with poor questions that don't help or benefit anyone but, at best, the person who asked the question.
This is NOT a replacement for the Help Center, Tour, or any other rule source
This is a convenience. It isn't the law. The law is found in the Help Center, Tour, VTC explanations, mouse rollovers, and wherever Stack Exchange chose to stuff things, as well as in the many Meta discussions that have established this Stack's unique policies. If there is any contradiction between this summary and the actual Rules, the Rules win.
I've included a few words of commentary here-and-there in block-quotes. These are my opinions based on several years of practice. Hopefully I've included relevant Meta links. If you disagree with my commentary, please post a comment to this Post and within a week I'll go chase down the relevant Meta discussions. In other words, if I've done my job right, I'm not just hoisting my opinion, I'm expressing real Policy perspectives that I didn't have the time to go research. Sorry about that....
There are a handful of "rules" that are really Stack policies that have been hammered out through months (and often years) of Meta discussions. Most of those are encompassed by the rules listed below and I've linked to a couple of them. But you will occasionally be directed to a meta post about a Stack policy as a justification for asking for your post to be improved or that it's being closed. I apologize that this summary isn't comprehensive — but welcome to Real Life. You can't find a comprehensive summary of your nation's Laws, either. You're expected to eventually do the research to catch up on your own. Nevertheless, as I or others come across those Policies and remember, we'll include them here.
- The question cannot be a duplicate of another question previously asked. (Source: "Duplicate" close reason)
If you believe your question is not a duplicate of a previous question, you are expected to explain why it is not.
The question lacks enough details or conditions for the average user to clearly understand the problem (Source: "Needs details or clarity" close reason)
The post is required to ask one and only one question per post. (Source: "Needs more focus" close reason)
As a tradition, additional questions may be asked, but you must state clearly that they are NOT expected as part of the answer, only "nice to haves." Answers not provided to secondary questions can NOT be used against users when judging a best answer. (Frankly, it's better to just ask one question.)
The question cannot invite too many answers. (Source: Tour)
This site has trouble with "fishing for ideas" questions, which are the most common type of question that completely breaks that rule. However, we respect the reality that when worldbuilding, sometimes what you need to see is a list of options. The bridge between the rule and the need are the concepts of an off-topic infinite list of things and an on-topic finite list of things. As you read through this list, you'll see that goals, expectations, restrictions, conditions, and limitations are insanely important.
Note that there is a contradiction with Help Center which also states questions should inspire long, not short answers. What this means is that a question easily answered with a one-sentence answer is a bad question. Likewise, a question that requires more then 4-5 paragraphs to answer is also a bad question.
- The question must be about the rules of your world, not a story based in your world or the actions/choices/development of any character. (Sources: "Too Story Based" close reason, Help Center)
Your world... not the Real World (see two bullets down), not a 3rd party or commercial world... not even mythology in any instance save the Anatomically Correct Series (which has specific rules). Worldbuilding is the design and consistent use of rules and systems for a fictional world of your own design wherein an infinite number of stories may be told.
- The question must refer to a specific, pracitcal issue with worldbuilding and be answerable. (Sources: Tour, Help Center, Help Center)
While we are somewhat lenient with this, questions must have an actual problem to be solved. Inviting Stack users to verify if something is believable, realistic, plausible, etc., is out-of-bounds as such questions are opinion-based unless specifically meeting the criteria of the reality-check tag. Note that asking if anything is realistic, plausible, believable, etc. in the Real World is strictly out-of-bounds. We're here to help you build your own, fictional world. Not to enhance the Real World.
- The question must be about worldbuilding (Source: Tour), specifically an imaginary world (Source: Help Center).
This touches on our Real World Question debate. Real World questions are permitted only if there is a dependency on your worldbuilding. As a specific example, a question about Real World history is only permissible when used as an example when building an alternative history or the history of a fictional world. (Source: Help Center)
The question must include context. (Source: Help Center)
The question must include restrictions and/or requirements. (Source: Help Center)
The question should include research. (Source: Help Center) Lack of simple research is a frequent cause for down voting questions (see down-vote mouse rollover text).
A question about an event must include the setting and circumstances of the event and the goal or result the question leads to. (Source: Help Center)
A question asking Stack users to "check" a condition (e.g., a "reality check") must include the relevant details (rules) of the OP's world (not the Real World) for the purpose of checking consistency. (Sources: Help Center, the reality-check tag wiki)
The question must be asked in a way that ever answer is NOT equally valid. (Source: Help Center)
What this means is the question must have a reasonable expectation of a best-answer selection (whether one is selected or not). This means that one answer above all must best meet the goals, conditions, and expectations set forth in the question. If the only criteria for selecting a best answer is the OP's fondness of one answer over another, the question is out-of-bounds.
The question must NOT have answers provided with it (e.g., "I've already considered and rejected the following..."). (Source: Help Center)
The question should inspire answers that explain "why" and "how." (Source: Help Center)
However, this Stack is much better at answering "how" than "why," which questions are often closed as opinion-based unless sufficient conditions, restrictions, and expectations are provided.
If your question meets all those expectations, you're in great shape! The more it misses, the more likely it'll get closed. And there's a few of them that'll get your question closed without discussion. Honestly, it's better to obey the rules.
1 This is just my opinion, but the "cause of worldbuilding" means you're asking questions that might actually have value to other people. That's actually one of the primary purposes of Stack Exchange as a whole. In an ideal world, no question can or should be asked that serves only one person's interest. If you're not working on problems for a specific worldbuilding project of your own design... if you're tempted to ask questions just because it's fun to ask questions or because, in one participant's words, you're only interested in "pure worldbuilding" (meaning you're not building a world, you're just asking random questions of interest to yourself)... please have the minimum courtesy of designing questions that really will benefit someone else's worldbuilding effort. That's actually harder to do than it sounds and while we're tolerant in the beginning, we do start down voting and closing questions from participants who don't grow out of breaking that "mindless social fun" rule.