Ideal observer theory is the meta-ethical view which claims that ethical sentences express truth-apt propositions about the attitudes of a hypothetical ideal observer. In other words, ideal observer theory states that ethical judgments should be interpreted as statements about the reactions that a neutral and fully informed observer would have; "x is good" means "an ideal observer would approve of x".
This makes ideal observer theory a subjectivist yet universalist form of cognitivism. Ideal observer theory stands in opposition to other forms of ethical subjectivism (e.g. moral relativism, and individualist ethical subjectivism), as well as to moral realism (which claims that moral propositions refer to objective facts, independent of anyone's attitudes or opinions), error theory (which denies that any moral propositions are true in any sense), and non-cognitivism (which denies that moral sentences express propositions at all).
