What does Genesis 4:15 mean?

In the previous verse, Cain expressed his fear over God's punishment. The murder of his brother would leave him vulnerable to being killed himself, likely in retribution. God, who is about to banish Cain from His presence, shows that He is still merciful—and determined to stop people from seeking revenge. So He promises Cain that He will take vengeance—times seven!—on anyone who kills Cain.

To seal the deal, and ward off all would-be attackers, God put a mark on Cain. We don't know what this mark looked like, or if it was even visual. All we know is that it communicated loudly and clearly to all who met Cain that God would take vengeance on anyone who killed the murderer of Abel.

Why would God do such a thing? Why not let Cain get what's coming to him? Later, God will build into the Law procedures both for bringing justice on wrongdoers and for helping murderers find sanctuary from those who would seek revenge. It seems God's purpose here is focused on preventing the never-ending cycle of revenge to which humans are prone. For now, God simply insists on being the one to take vengeance on injustice. In the New Testament, the Apostle Paul will reveal that this is still a role God demands to play today (Romans 12:19).

This verse also strongly suggests that there were other people alive on earth other than Adam, Eve, and Cain. More than likely, this incident occurred many years after Cain and Abel's birth, perhaps many decades later. Though the Bible does not explicitly mention them, Adam and Eve probably had other children during this time. As mankind "multiplies" and the earth is filled, there are enough people for Cain to be concerned over.