StackExchange sites work on the basis of community moderation: as a user gains reputation through upvoted questions and answers, moderation powers increase: http://mathoverflow.net/help/privileges. What has happened is that five users who have earned more than 3000 points of reputation have voted to put the question on hold. As noted by angussidney above, the users in this case are not the same as site moderators, who are elected to the position by the community and who are involved in day-to-day maintenance of the site among other things: http://mathoverflow.net/help/site-moderators. Site moderators have a diamond next to their usernames.
Having a question put "on hold" is not necessarily the end of the story; such decisions can be and are often reversed. But usually this will involve some effort by the OP to improve the question (improve the clarity, provide context and/or motivation, link to relevant sources, etc.), and if you strongly believe that your question is on-topic for this site (a site which serves professional mathematicians and their graduate students), then you should probably put in such effort.
Users make their best judgment as to what is on-topic for this site (e.g., what is "research-level"), but on occasion the judgments may be based on impressions or intuitions as opposed to deep engagement with the subject matter. If however the OP makes an effort to formulate a question with a professional appearance, and is willing to engage with users who state their criticisms, the more superficial reasons for closure will likely not be valid. We also have a meta post for requests to reopen questions, usually by other reputable users who call into question the decision to close: Requests for reopen and undelete votes for closed and deleted questions.
