There are two lines in the FAQ that are seemingly contradictory, both in the What kind of questions should I not ask here? section, and a third reference to this problem in the Stack Exchange about page's What’s special about Stack Exchange? section.
As Matthew Read said in the comments, Statement I allows for curiousness-driven questions to be posted on Stack Exchange sites (and I think it's safe to say that they must also meet the other criteria for an allowable question). Statements II and III do not allow for such questions, and require the motivation behind a question to be solving a particular problem.
Do you, or do you not, need a concrete problem that you are trying to solve in order to pose a question to any single Stack Exchange community?
However, if your motivation is “I would like others to explain __ to me”, then you are probably OK.
there is no actual problem to be solved: “I’m curious if other people feel like I do.”
Statement IIa (Programmers.SE Moderator Mark Trapp) (I think this got deleted in something of an insanity spree in what gets moved/posted where, but I believe Mark can confirm this statement):
If the question askers provided a "This is my problem, this is what I'm thinking, how can I solve it?" story, they'd be fine. That's all we're looking for: some concrete way by which to evaluate the answers.
We also expect questions to represent real problems, not just imponderables, hypotheticals, or requests for opinions.
To make this concrete, let's just take a question from Stack Overflow: Why MutableString is deprecated in Python? This question has 2 upvotes, 0 downvotes, and over 330 views. It's an OK question, and something that I'm sure people have wondered. However, it doesn't solve anyone's problem. Does that mean this question should be closed?
Maybe I'm doing a really crappy job explaining my confusion.
What is the difference between asking someone to explain a concept that you read in a book (allowed, per Statement I) and asking a question that doesn't solve an actual problem (not allowed, per Statements II and III)? To me, they are the exact same thing. Not understanding a concept would potentially lead me to not being able to solve a problem, but until I try to apply the concept to a concrete solution, then it's not a problem.