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.
keywords or labels that categorize and group your question with other, similar questions.
Questions about the scope of the site: what's on or off topic, etc.
Questions about how to promote Chess SE, in order to attract more users.
For questions about the bounty system, which allows users to spend some of their reputation in order to draw more attention to a question.