Suppose one person says '3,277 is a prime number,' while another says '3,277 is not a prime number.' Since these two people have opposite views, it seems pretty clear that one of them has to be wrong. There's just no way that they could both be correct.
But now consider a different kind of case. Suppose that one person says 'It is horribly wrong to commit murder,' while another says 'Murder isn't really that bad -- in fact, it's completely fine.' In a case like this one, does one of the two people have to be saying something incorrect?
Philosophers have long assumed that people see morality as objective, indeed, that they think of moral truths as being just as objectively true as the truths of mathematics. So philosophers typically assume that people would think that if there is a disagreement between these two moral views, one of them has to be incorrect.
But recent work in experimental philosophy seems to be challenging that traditional picture. This work suggests that people don't actually see morality as objective -- that people might be moral relativists.