Metaethics is the study about Ethics. I.e, answering questions like "what is ethics?", "do moral truths exist, objectively, absolutely, mind-independently, or are they mere expressions of sentiments, espousals of ways of living?", "why should we trust our moral intuitions?", "does moral philosophy rest on a mistake?", "how do we get in touch with moral truths, if they exist?", and questions about these questions. 

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Further Reading: 
"Metaethics", Stanford Encyclopedia of Philosophy