“Karma will get him back one day.”
What does this phrase mean? What exactly is Karma?
Karma is the idea that actions we take create more actions. It is not universal justice (as this idea does not exist yet).
Generally, someone who takes bad actions will eventually create / face bad actions.
However, have you ever met someone who is a terrible person, who never seems to face the consequences. Well, his/her karma is that he/she will never experience the results of good karma. The mother who berates her children will get disciplined children (neutral karma), but she will never experience loving children (good karma).
Of course, life is not black and white. Good people can get bad karma, and bad people can get good karma. Additionally, we are not entitled to know the consequences of our karma.
[Recommended reading: The Bhagavad Gita]
[Recommended historical perspective which shows one individual’s karma: The Code of Hammurabi by L. W. King]