A request to reopen a *specific* closed question, or receive further explanation of why it was closed. If your request is to reopen a closed question, it should be one here on Meta Stack Exchange. If it's on another site, ask it on the per-site meta instead. For questions about the reopen process itself, use the [vote-to-reopen] tag.
Officially, this tag should be used to request a closed question be reopened by the community. Unofficially, this tag can be used to receive a further explanation of why the question was closed in the first place, since that is most likely the response to an invalid request at reopening a closed question.
If you are requesting to have a closed question reopened, it must be one on Meta Stack Exchange. Use it along with the discussion and meta tags and post your request. If the question is on another site, ask it on that site's meta rather than Meta Stack Exchange.
For questions about the reopen process itself, use the vote-to-reopen tag.