It is basically wishing sinners well as they are about to die, and will have to make amends for their sins.
What does "Blessed Be The Sinners, for the day of atonement is at hand" mean?
It is the threat of the impending mythical "Stick" in the Christian belief's "Carrot and Stick" or "Reward and Retribution" pitch to make the decision to follow (believe in) THEIR God . . . Versus the many others.
The threat of "Reaping what you Sow" in this life transfers over to eternal Payment in regards to the concept of an After life.
Heck, even Buddhism believes your actions in this life govern the type of life form you are reincarnated into as Payment or Punishment.
(EDIT to Original post)
Atonement actually refers to the reconciliation of Man and God, through the sacrifice of Christ. It is one of the key elements in the concept of "Grace" in that man is undeserving of this level of forgiveness.
I'll save my comments of how ridiculous the concept of Grace is in this regard, for another time.
Actually it refers to the doctrine in theology concerning the reconciliation of God and humankind, especially as accomplished through the life, suffering, and death of Christ.
It's why we refer to original sin as a "happy fault." The redemption wrought by the God-man has elevated those in grace above what would have been due to the just if Adam had not fallen.
(Not my words, but they are precise and from two different references.)