This tag is for use in questions pertaining to one question on the main site in particular.
As opposed to the questions tag, this tag is for discussion or support about a single, specific question from the main site. It should be used, for example, when you ask about why your question was closed, or whether this question really is on topic.
As a general rule, questions on meta with this tag should not also have the questions tag and vice versa. However, this rule has some exceptions, so use common sense when tagging. If you believe your question might have an answer applicable to many new questions on the main site, tag it questions also, but keep the specific-question tag, as it should still pertain to one question.
For example, the question linked above (this one) might be tagged both, as although it related to one question to start with, the answers have given rise to some discussion about what is on-topic here, a subject applying to all new questions.