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 whether a question is on topic and what to do with off topic questions.
For questions about the MTG autocard feature, which converts [mtg:CardName] tags in posts into Gatherer card links.
for discussing questions related to software, hardware, and computer stuff in general on the main site.
the answer the question owner thought best met their needs. This tag is for discussing issues related to accepted answers, not indicating that a question has an accepted answer.