What does Genesis 4:10 mean?

After Cain denied knowing where Abel was, God reveals that He already knows. Using poetic language, God says He can hear the voice of Abel's blood crying to Him from the ground where it was spilled. How Cain attempted to cover up his crime, we don't know. Perhaps he buried Abel or he just left him laying somewhere. In any event, beyond God's own omniscience, there would have been some physical traces left to connect Abel's murder to Cain. No part of Cain's denial will work, either against God or against other men.

This first murder begins to reveal God's principles of both justice and mercy. Blood spilled in unjust death cries out to be avenged, for the wrong to be made right. God will take responsibility for executing that justice, as well as for making a way for some amount of mercy to be shown to the sinner. As with Adam and Eve, God chooses mercy and exile over destruction.