“An example will perhaps better illustrate these points. A boy playing ball breaks a window of his home. Contrite and sorrowful he goes to his father, who forgives him. However, despite the forgiveness the window is still broken and must be repaired. Since the boy’s personal resources are insufficient to pay for a new window, the father requires him to pay a few dollars from his savings and forego some of his allowance for several weeks, but that he, the father, will pay the rest. This balances justice and mercy (generous love). To ask the boy to do nothing, when it is possible for him to make some reparation, would not be in accordance with the truth, or even the boy’s good. Yet, even this temporal debt is beyond the boy’s possibilities. Therefore, from his own treasury the father generously makes up what the child cannot provide. This is indulgence. Unlike the theologies that say “we are washed it the blood of the Lamb and there is nothing left to do,” Catholic teaching respects the natural order of justice, as Jesus clearly did in the Gospels, yet recognizes that man cannot foresee or undo all the temporal consequences of his sin. However, God in His mercy will satisfy justice for what we cannot repair.”

to read the complete explanation go to: https://www.ewtn.com/devotionals/mercy/what.htm

Pk – I had to look up what an indulgence is too. Here’s what I found in Wikipedia taken from the Catechism of the Catholic Church. Hope this helps.
The Catechism of the Catholic Church describes an indulgence as “a remission before God of the temporal punishment due to sins whose guilt has already been forgiven, which the faithful Christian who is duly disposed gains under certain prescribed conditions through the action of the Church which, as the minister of redemption, dispenses and applies with authority the treasury of the satisfactions of Christ and all of the saints”.