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.
For questions regarding the nature of a "question" on StackExchange sites; e.g. how to ask, whether specific questions are appropriate, etc.
when examining what is on-topic or off-topic, and why.
the process of migrating questions from one Stack Exchange site to another.
For questions about the syntax highlighting performed inside code blocks on Emacs Stack Exchange.
For questions about the /review path where users can view and act on posts by other users that the system thinks may need attention.