It's probably worth starting a few meta threads to discuss and debate policy for closing questions, even though this is -- as MO experience suggests -- likely to evolve over time. Note that typically it is up to the community's high rep users and not the moderators to close questions (except in cases of actual spam or offensive material), because one moderator vote will close the question, and usually they prefer to avoid that lest they contradict the community consensus.
This thread is motivated by the question "What is the meaning of this symbol?" I think that it is a good example of the kind of question that should be closed as "too localized." I hope, incidentally, that the OP does not take this personally --- we're all trying to figure out what works and doesn't work here, and this is something that usually takes a while to reach a community consensus. The reason is that simple definition questions can just be looked up. In addition, there isn't any context for why the OP was interested in the question, say a reference he was reading.
Homework questions also make good candidates for being closed in this manner, though policy on them has a separate meta thread.
If the question were instead, "I'm having difficulty understanding the meaning of entailment, please explain" (assuming the OP came across the symbol in the context of logic), I think it would be a much better question. This is specific while yet a simple one-word answer or Wikipedia reference does not answer it.
In general, while SE sites encourage specificity, it is still useful to tell a story behind one's question and explain one's motivation.
I've started this thread for us to establish policy on the "too localized" option.