Prayer makes reparation, especially when done before the Blessed Sacrament for all the outrages this Divine Heart has received from men and women, even from those who are especially consecrated to his service. Reparation is like paying a debt, a payback, a return of something that is owed to another. In theology it refers to a relationship one has with Christ in that you want to make up for the inadequacies of someone else's debt to God. In that respect, reparation is a participation in the redemptive action of Christ in His Passion. The person through love makes up to Christ not only for something that happened 2000 years ago on Calvary, but for the unlove which is happening this day and time.
Atonement is much the same.
Some might say that since Christ is God and now risen in glory, He does not need now our love and reparation. This is not the thought of the Catholic Church. Christ has a human Heart which knows and feels the lack of love even in a glorified state. We cannot deny Christ anything which a human heart now feels, otherwise He would not be man. Our love and reparation has brought us a long way, and yet because we are dealing with a divine mystery, we have further to go.