In the teaching of the Roman Catholic Church, an indulgence is “a way to reduce the amount of punishment one has to undergo for sins”.^[1]It may reduce the “temporal punishment for sin” after death (as opposed to the eternal punishment merited by mortal sin), in the state or process of purification called Purgatory.

https://en.m.wikipedia.org/wiki/Catechism_of_the_Catholic_Church
https://en.m.wikipedia.org/wiki/Indulgence