The A to Z of isms, relativism, moral relativism is the view that what's right in one culture may be wrong in another and that there's no way of judging between the two. It's all relative. There's no absolute truth of the matter. Different strokes for different folks. And that's OK.
Imagine a Martian landing on earth.
She would see that people behave differently in different places. They've got different moral rules, different social customs. What's right in one location is wrong in another. She might well conclude that there's no such thing as the right way to live or the right thing to do for everyone at all times.
It's all a matter of cultural norms.
So in parts of Spain, bullfighting is seen as a spectacle, whereas in many other countries it's considered outrageous cruelty. Try setting up a boring and present day London, for instance.
If the visiting Martian could get an overview of the history of the world, she'd see that in ancient Rome it was fine to have gladiatorial combat to the death and to own slaves. But the best practices would be widely frowned upon in present day Rome or Amsterdam or elsewhere. Attending fights to the death and keeping slaves was acceptable for the Romans, but would be wrong in today's Europe. This sort of relativism has been popular with some anthropologists who've attempted to appreciate other cultures from within understanding sets of interwoven practices as responses to particular circumstances and traditions.
They recognize that morality is different in different cultures, and they often argue that it's ethnocentric to impose your own set of values elsewhere.
As if you know best. Such nonjudgmental relativism might seem attractive.
But think about Nazi Germany. There he became socially acceptable and even required to treat Jews, homosexuals and gypsies as less than human. This became right there, both legally and morally relativist would simply have to bite the bullet and say that that was indeed right for Nazi Germany. That's what morality meant there. And that is morality, which treats everybody with equal respect is no better or worse than the Nazi way of doing things just different. Unless you want to embrace that sort of repugnant conclusion, it's probably best to steer clear of moral relativism.
Some people go further and say that everything is relative, even truth, that what is true at one time in one place isn't true for everyone everywhere, or that there is no such thing as objective truth, just the truth. According to me, we see hints of this in politics today. There is a big problem with this sort of relativism, though the theory of relativism itself would have to, on its own account, be relative. So the theory that truth is relative couldn't be absolutely true.