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 about the Winter Bash (or holiday hats) to celebrate the end of the (Gregorian) year.
for questions about suspected user spam, the spam flag or combating spam as a community.
for questions about Mi Yodeya's publication projects in general as opposed to specific projects.