An often-cited quote from the Torah. It appears, among other places, in the book of Leviticus, chapter 24, verses 19-20:
If anyone injures his neighbor, whatever he has done must be done to him: fracture for fracture, eye for eye, tooth for tooth. As he has injured the other, so he is to be injured.

Jesus took issue with the sometimes excessively liberal interpretation Jews of his day took this verse. In context, it was always used to determine judicial punishment for a certain crime: the same severity should be shown to the one who committed the crime, but the punishment should also be no more severe that the crime committed.

But in the Sermon on the Mount, Jesus challenged the Jews to reconsider it in a personal context: "If someone strikes you on the right cheek, turn to him the other also" (Matthew 5:39), and soon after, "Love your enemies and pray for those who persecute you" (v. 44). Indeed, Mahatma Gandhi knowingly used this principle when he initiated nonviolent protest against the British domination of India.

The idea is that punishment and restitution for crimes committed should be taken care of by the courts, by the governments, and by God, not by individuals.