God, in Genesis 3:15, says that the offspring of Satan would be at enmity, or in opposition to and hatred of the offspring of Eve. God also said that Satan would bruise the heel of Eve's offspring, but that her offspring would crush Satan's head. The offspring of Eve would be Jesus. He would crush the head of Satan, meaning Jesus would defeat him at the cross and through his resurrection.
The human race, represented by Adam, had walked away from a love relationship with God. The penalty for this act was death.