The basic idea of ethical relativism is that there is nothing that is absolutely ethical or unethical. Instead, actions are ethical or unethical only in a given context. Usually, this context is a cultural one. To someone who believes in ethical relativism, an action is only ethical or unethical if the ethical values of a given culture say it is.
In this way of thinking, there is no such thing as an absolute moral truth that holds for all people in all societies. It is not correct, for example, to say that women have certain rights that must be protected in all ethical societies. Instead, we can say that actions that would be considered abusive in the United States (denying women the right to drive or to make most decisions without a male guardian's approval) can be perfectly ethical in Saudi Arabia because that society has a different moral and ethical set of standards.