Meta-ethical naturalism refers to meta-ethical theories which hold that moral terms such as “good” and “bad” and the entire domain of morality and ethics can be reduced to one or another observable properties of nature. A concrete example is meta-ethical hedonism, or the view that moral terms are ultimately reducible to pleasure (whether understood as the mental feeling of happiness, pleasant physical sensations, or both) and that moral statements such as “that action is good” are reducible to apparently empirical statements such as “that action leads to pleasure.”
If this is the case, there is no way to reason from the fact that a certain action leads to pleasure in one of the senses described to the belief that there is a duty to engage in that action because it leads to pleasure. The same holds for all attempts to derive moral duties from observable properties of nature. Thus, ethical naturalism is an unsustainable viewpoint.