In addition to @Dom's OP-oriented post, I'd like to add a rest-of-the-community-oriented post:
Vote early; Vote often!
SE is built to democratically decide which questions are on/off topic by its voting mechanisms and requirement of multiple votes (with exceptions) for an action to be taken.
The community can vote to "close" or "leave open". The community can also vote to "reopen" or "leave closed". Mods can override the community, and the community can override mods.
Don't be shy about voting. Remember, you aren't making the decision; you're just expressing a point of view. The rest of the community is free to agree or disagree.
"Closed" does not mean "Dead"
"Closed" means a question it doesn't fit site guidelines in its current state. But not only can the OP edit their question and request reopening, anyone (with enough rep) can edit the question and request reopening.
Even "Deleted" questions can be undeleted and reopened, though the bar is higher.
Edit, edit, edit!
Sometimes titles are misleading. Sometimes questions are poorly articulated. Don't be shy about editing. Your edits can be rolled back if you miss the mark, but more likely you're helping someone get their question answered quickly and accurately, rather than closed.
Retract your votes
Sometimes you might vote to close (for example) a question that gets improved before actually being closed. You can take back your vote.
The whole point of a vote is to say "this question currently meets (or doesn't meet) a certain criteria." If that situation changes, you can reflect that by unvoting.
The moderators are very experienced. If you feel uncertain about a question but don't feel it clearly demands a vote one way or the other, point it out to the mods by raising a flag and explaining your uncertainty. They're response will give you an indication of how to handle similar questions in the future.
The whole point of Meta is to discuss the site itself, including how to handle particular content. Similar to raising a flag, this allows the whole community to weigh in on the issue.
See "Use Meta".
And did I mention VOTING! VOTING! VOTING!