What the word means
Expiation is a theological word, rather than a word commonly used in the KJV. It means that sin is covered, cleansed, or taken away so that its guilt no longer stands against the person who receives God's forgiveness. The KJV often speaks of making atonement, especially in the law given to Israel.
On the Day of Atonement, Israel's high priest carried out sacrifices connected with the cleansing of both the people and the sanctuary. The point was serious but hopeful: sin damages fellowship with God, yet God himself provided a way for his people to be cleansed (Leviticus 16:30).
For on that day shall the priest make an atonement for you, to cleanse you, that ye may be clean from all your sins before the Lord.
More than ignoring wrong
Expiation does not mean God simply overlooks evil or tells people that wrongdoing does not matter. Sin is real, and Scripture treats it as something that brings guilt and death. Under the old covenant, blood sacrifices taught Israel that forgiveness was costly and that life was required in dealing with sin (Hebrews 9:22).
Christians sometimes distinguish expiation from propitiation. Expiation emphasises sin and guilt being removed. Propitiation emphasises God's righteous anger against sin being dealt with. The two ideas belong closely together in the Bible's picture of Jesus. Romans 3:25 calls Christ a propitiation through faith in his blood, showing that God remains just while making forgiveness possible (Romans 3:25).
Whom God hath set forth to be a propitiation through faith in his blood, to declare his righteousness for the remission of sins that are past, through the forbearance of God;
Jesus fulfils the sacrifice
The animal sacrifices in Leviticus were never presented as the final answer. They pointed ahead. Jesus offered himself willingly, and the New Testament says that believers are sanctified through the offering of his body once for all (Hebrews 10:10). His death was not a repeated ritual that needs topping up. It was God's decisive provision for sin.
That means expiation is not something you achieve by becoming good enough. We respond by turning to God in faith, trusting what Christ has done. When Scripture says that Jesus is the propitiation for sins, it directs our hope away from our own efforts and towards him (1 John 2:2).
What it means personally
If you carry regret, expiation says your sin need not have the final word. God does not ask you to deny it, hide it, or repair your standing with him by yourself. He calls you to confess honestly and receive the cleansing he gives through Christ (1 John 1:9).
This is also why Christian forgiveness has weight. God is not casual about evil, but neither is he unwilling to welcome the repentant. Read the accounts of Jesus' death and resurrection in the Gospels, and keep asking how his sacrifice changes the way you see God, sin, and grace.