An indulgence is is a removal of all or some of the punishment due to already forgiven sin because of the performance of a good deed or the saying of a prayer.
This is an issue Luther used against the Catholic Church in his 95 Theses. Specifically, because the Pope had granted a plenary indulgence to those who would donate money toward the building of the Vatican [St. Peter's]. Then as now, many didn't understand the teaching and believed Luther when he falsely accused of us "selling" the indulgence to gain access to heaven.
I disagree with Luther's view and condemnation of indulgences. He said that we could never do anything to remove the punishment we deserved. I believe God is loving and merciful to those who repent with a sincere heart.
Thanks for scanning my links.