There are many different types of meta-ethical theory. One useful dichotomy for classifying the theories is "descriptivism" vs. "non-descriptivism", a distinction recognised by RM Hare in his book "The Language of Morals" and later works. An example of ethical descriptivism is "naturalism", which asserts that moral value is a real, objective property of the natural (physical) world - hence a moral judgement is either true or false, depending on the facts of the world as it exists. A most fascinating exposition of naturalism is given in David Pearce's Hedonistic Imperative (sections 2.11 to 2.20) which gives a utilitarian account of value naturalism in terms of brain states.
This type of theory forms part of the "Is" side of the meta-ethical debate. The other side of the "Is-Ought" debate maintains that moral judgments are not truth-evaluable at all - they do not describe any facts of the world, therefore they cannot be accurate or inaccurate descriptions. (See the IEP on noncognitivism.) This type of theory draws a sharp distinction between facts and values (or "evaluative" judgements).
One type of non-descriptivist theory is "prescriptivism". Hare (a prescriptivist) would produce an argument against naturalism something like this: To commit to a naturalistic utilitarian position is to define value (and therefore good) in terms of brain states. But having done this, to then say that "happiness is valuable" would be to affirm a mere tautology, as would "happiness is good" - in neither case would we be commending happiness, which is something (Hare maintains) a utilitarian would want to do. All we would've done is give these brain states the names "value" and "good", in addition to "happiness", so we would not (in later saying "happiness is good") have given anyone a reason (or even a request) to promote the existence of these brain states. If we accept that "happiness is good" has some prescriptive content, i.e. that it (in some way) implies or at least suggests "let there be more happiness", then we cannot allow "good" to be wholly descriptive... it cannot simply the name of some brain states or natural properties.
Such objections have been considered by Peter Singer (1973). Singer agrees that by binding moral judgements so closely to the facts of the world (so that a description of the world directly implies its moral status), it becomes quite unclear how the moral judgement is related to actions or prescriptions - it is not clear why then a moral judgement should guide our actions. However (Singer notices), if we make moral judgements prescriptive so that they are closely related to and indeed bound to the task of guiding actions - in a way expressing some command - it then becomes unclear how these moral judgements are related to the facts of the world: for a command to do this or that can be expressed no matter what the facts about this or that (or the consequences or doing this or that) are. Naturalism leaves a "gap" between moral judgements and prescriptions for action, but prescriptivism leaves a "gap" between moral judgements and the facts of the world. Neither account can make the transition all the way from an analysis of the world, to a moral judgement, to a prescription, without some leap over the gap - and if there must be some gap, it is not obvious that it really matters where this gap in the theory is to be... which is why, Singer declares, the debate over "Is-Ought" is essentially trivial.
See also "The Naturalistic Fallacy: The Logic Of Its Refutation" by A. N. Prior.