Most ethical theories, whether they judge actions by their consequences, by duty, or by character, operate at what philosophers call the normative level, offering an account of which specific actions are actually right or wrong. Metaethics operates one level beneath all of that, asking a more fundamental question those normative theories generally take for granted: whether moral claims can be objectively true or false at all in the first place, and if they can, what actually makes a given moral claim true rather than false.
Normative ethics assumes an answer metaethics is the field that actually asks
A normative ethical theory, one arguing that lying is generally wrong, or that an action's moral status depends on its consequences, typically presupposes that moral claims are the kind of thing that can genuinely be true or false, the way factual claims about the physical world can be. Metaethics steps back from any specific normative claim and asks whether that underlying assumption is actually correct: are moral claims genuinely capable of objective truth, the way "water boils at 100 degrees Celsius at sea level" is, or are they instead expressions of something else entirely, personal or cultural preference, emotional attitude, social convention, that merely resemble factual claims in their grammatical form without actually functioning like one?
Different metaethical positions locate moral truth in genuinely different places
Philosophers have staked out substantially different metaethical positions on this question. Moral realists hold that moral facts genuinely exist, independent of what any particular person or culture happens to believe about them, in a way roughly analogous to how mathematical or scientific facts are typically held to exist. Various anti-realist positions instead argue that apparent moral claims don't actually describe objective facts at all, understanding them instead as expressions of attitude, as social constructs, or as claims whose truth is inherently relative to a particular cultural or individual perspective. Which of these metaethical positions is correct directly shapes what it would even mean for any specific normative ethical theory to be right in the first place.
What we're still unsure about
The basic distinction between normative ethics and metaethics, and the range of metaethical positions philosophers have proposed, are well documented, foundational moral philosophy. What remains a genuinely unresolved, actively debated core question in the field is which of these metaethical positions, if any, is actually correct, since philosophers have not converged on a single settled answer despite centuries of sustained argument — this isn't a case of the field having simply not yet caught up to an obvious answer; the disagreement concerns what would even count as adequate evidence for settling a question of this fundamental kind, which is exactly why metaethical debate has remained genuinely live for as long as it has.
This sits inside Metaethics: The Nature of Moral Facts, one of seven topics in Ethics, one of five domains in Philosophy, one of seventeen subjects the app can quiz you on.