Morality As a Limit
Moral Relativism is an ethical judgment. It is the claim that no ethical system is better than another. It stems from the fact that to judge an ethical system, it must be judged by a moral standard. Since every ethical system should evaluate itself as the best and only moral system, and every other system is flawed and immoral, it is assumed that moral judgments about ethical systems are meaningless.
Moral Relativism rests on the belief that values are subjective. It is holds the belief that there is no objective morality. That there is no such thing as right and wrong, good or evil. Only if moral systems are just made up and supported only by personal or social bias can moral judgments of ethical systems be wasted. Moral Relativism cannot and does not accept the possibility that an objective moral system exists. If it did, you could evaluate other ethical systems meaningfully.
Moral Relativism is the denial of truth in ethical questions. A Moral Relativist accepts that his own moral system is meaningless and is accepted on whim, not reason. Intellectually, Moral Relativism is an attempt at destroying the concept of ethics. This is done by claiming that ethics are irrelevant and we accept them due to societal conditioning. Since morality is necessary, Moral Relativism is a default on the responsibility of choosing a rational moral guide.
Since Moral Relativism holds that ethical systems are subjective, it claims that none is better or worse than another. It makes an exception, though. Any system that claims to be true or absolute is evil. It commits the only possible truly evil act. It maintains the possibility of truth in ethics.