When you sin and go to confession, your sins are forgiven, but you are still subject to penance (for example, 10 rosaries).
As an indulgence, you can offer to do a good deed (maybe work in a food shelter for an afternoon) in place of this penance.
Now let’s say, I died and I’m in Purgatory. You can do a good deed which will help lessen my time in Purgatory.
Simply put, Indulgences are meant to lessen the punishment that we have to pay (penance or purgatory) for sins that already have been forgiven.
Indulgences DO NOT BUY FORGIVENESS FOR SINS, they only lessen the punishment.
Many people mistakenly think that indulgences help buy forgiveness. If that is so, then shame on the priest that allowed that to happen. I’m pretty sure there were some priestly abuses of indulgences in the past (this was one of Martin Luthers more legitimate gripes), but the priests were abusing them outside of the Church’s Teachings.
In other words, there were some weeds in the wheat.
Still simple, huh?