system
Given the following post, respond as if you're commenting on Reddit:
human
What does it mean if "morality exists" (i.e. if moral realism is true), from an atheist perspective?

Morality is essentially a value judgment: this action is good, another one is evil. Something "should" be done. 

If you're religious, this makes some sense: These rules were created by God and, according to some beliefs, if you follow them you get rewarded. 

But what does "morality being real" mean if you're an atheist? To put it differently: What is the difference, in practice, between moral realism and moral non realism? What does it mean to violate real moral norms if there is no higher entity nor afterlife? Yes, immoral behavior can have social repercussions, but these exist irrespective of the objective existence of morality.
gpt
It means there's an objective fact that you erred if you do something wrong, the same way there's an objective fact you erred if you got a math problem wrong, but there are no necessary external consequences, just like getting a math problem wrong.

It should also be made clear though that you're also missing the point if you only focus on bad consequences of doing wrong in the case of theism being right, the actual point is the same as in the atheist case.