Relativism is the belief that there's no absolute truth, only the truths that a particular individual or culture happen to believe. If you believe in relativism, then you think different people can have different views about what's moral and immoral.
Understandably, relativism makes a lot of people uncomfortable. Is it morally okay for some cultures to have slaves, for example? Cultural relativists might argue yes. Einstein, in particular, caused a lot of problems with his Theory of Relativity that stated that even time and space were relative concepts depending on where an observer happened to be. Of course, you might argue that relativism itself is a belief only a certain individual or culture happens to believe in. Tough stuff, philosophy.