A tag is a keyword or label that categorizes your question with other, similar questions. Using the right tags makes it easier for others to find and answer your question.
Questions regarding tools only available to users that have reached the 10k reputation mark
Any concerns about the scope of the site, as explained in the FAQ
Anything related to posting answers, e.g. about improving their content, voting on answers, editing others' answers, etc.
Something amiss or broken with how you're working with the site? Report it.
Anything about questions that were closed by community or a moderator
Questions about reasons for closing a specific question (use [specific-question] too), and the predefined reasons users are able to choose from when voting to close
Comments that can be added to questions or answers
Moderator-only tag for Community Promotion Ads questions.
For questions about the user named Community ♦ present on Stack Exchange sites.