(Genesis 4), God cursed Cain for murdering his brother Abel. The punishment consisted of two parts: Cain’s agricultural livelihood was ruined because the ground would no longer yield crops for him, and he was exiled to wander the earth as a restless fugitive. 

Cain feared his exile would make him an easy target for vengeance, so God mercifully gave him a “mark”. This mark was not a punishment, but a protective sign warning others that God would severely punish anyone who killed Cain.