It's partly the rules of the society as defined by the people in charge. A good example of this might be right and wrong as defined by a religion (religions simply being mechanisms of social control). Something might be 'right' simply because it is more conducive to harmony in a society comprising many individuals, or it might be deliberate behaviour-modification with the establishment of negative consequences for those who transgress.

Really it's a complex subject with no single clear answer. You can go back much further, and consider a default right and wrong as being partially something innate, distilled over millions of years... we might consider evolutionary selection pressures in favour of reciprocal altruism as an example in this case.

And on top of this, there is undirected drift, through changes in culture, homosexuality as an example again here, which in many societies is considered very differently by younger people today than it was even 20 or 30 years ago.