It's the notion that all forms of morality are plausible, not just ours(moral absolutism), or the ones we are willing to accept(moral pluralism). There are no universal standards of good and evil, right and wrong, but whatever we devise on our own.
Moral relativism is a term that describes the idea that people's understanding of morals, ethical standards, what is right/wrong, etc. is shaped by their culture, life experiences, education, and so...