The Word "Propitiation"
The word, "propitiation" is incredibly important in the Bible, and therefore, there is a ton to say about it. But this is a micro article! I only want to give the definition so that, if you see or hear the word in one of our resources, you have quick access to its biblical meaning.
Propitiation (noun) - The act of turning away God's judgment (i.e., his wrath against sin) by a blood sacrifice.
Jesus' death on the cross was a propitiation. He, having lived a righteous life (1 Peter 3:18), offered Himself as a substitute (i.e., in the place of others) sacrifice who received the judgment that others deserved. Jesus, in receiving the judgment we deserved, turned it away from its rightful destination, us. Jesus' death is a great mercy toward sinners, sparing us from divine judgment. Jesus' death, in other words, is what saves sinners and makes them right with God.
Jesus' propitiation on the cross is the way our sins are forgiven. It is the climactic act of God's love:
In this is love, not that we have loved God but that he loved us and sent his Son to be the propitiation for our sins.
1 John 4:10
Propitiation is the heart of the good news of Jesus:
For all have sinned and fall short of the glory of God,  and are justified by his grace as a gift, through the redemption that is in Christ Jesus,  whom God put forward as a propitiation by his blood, to be received by faith. This was to show God's righteousness, because in his divine forbearance he had passed over former sins.
Truly, Jesus' death on the cross is good news of great joy for sinners who, otherwise, have no hope of forgiveness. His death turns away the wrath of God that we deserve so that we never have to face it. His death is a propitiation. We, to share in Him and to share the blessings of His death, must turn to Jesus in faith. Faith is the heart trusting in Jesus alone for eternal life, i.e., the forgiveness of sins and a relationship with God that will never end and that no one can take away.