When you read the Bible or study about God, you learn that some things are wrong for you to do. It could be, telling a lie, cheating in a game or at school, stealing from someone, or some other bad thing. God has told you not to do these things, but sometimes you do not obey what He has said. This is called "sin."
God knew we needed to be rescued from our sins. He sent His Son, Jesus to save us. Jesus was God's only Son, and He did not sin. Jesus took the punishment that God was going to give to us when He died on the cross. Jesus is our wonderful Valentine from God!