"And I will put enmity
Between you and the woman,
And between your seed and her seed;
He shall bruise you on the head,
And you shall bruise him on the heel.'"
"Therefore when Jesus had received the sour wine, He said, 'It is finished!' And He bowed His head and gave up His spirit."
The word "finished" in Greek has three senses according to the lexicon:
1) to bring to a close, to end
2) to perform, fulfill, accomplish (so that the thing done corresponds to what has been said, the order, command etc.)
3) to pay (to pay a tribute)
When Adam and Eve sinned against God's word, they introduced sin into mankind. Since then, each and every person sins. As Romans 3:23 says, "all have sinned and fall short of the glory of God". Because God is holy, He must punish sin and sinners. Yet God is also loving. Knowing that each man's righteousness will never be enough to meet His requirement, God provided His own Son as a substitute to pay the penalty that our sins deserved. He promises the Savior in the third chapter of the Bible. 1 Peter 2:24 says that Jesus "bore our sins in his body on the tree". In doing so, Jesus paid the penalty for our sins and God's righteous requirement was met by his sacrifice. For those who put their faith in Jesus, Jesus' sacrifice pays the debt that their individual sins owed and they are declared righteous in God's sight. Romans 8:33 says that "...It is God who justifies." And Ephesians 2:8 says that "...by grace you have been saved through faith, and that not of yourselves; it is the gift of God". The book of Romans is a very good place to read about Salvation and what it means to be a believer in the Savior.
"For God so loved the world that He gave His only begotten Son, that whoever believes in Him should not perish but have everlasting life. For God did not send His Son into the world to condemn the world, but that the world through Him might be saved." John 3:16-17
"But He was wounded for our transgressions,
He was bruised for our iniquities;
The chastisement for our peace was upon Him,
And by His stripes we are healed." Isaiah 53:5