Think of tags as question categories or topics. When you ask a question, you have to provide at least one tag for that question.

Tags allow us to categorize questions (and show related questions) without forcing the categories into a hierarchy. You can tag one question climate and policy, another question climate and adaptation, and a third question low-carbon and policy. You can then choose to ignore certain tags, like low-carbon, but mark other tags as interesting, like policy. In that case the site will start to show you more policy questions and less low-carbon questions.