An indulgence is the removal of some or all of the temporal punishment due for sins already forgiven.
Every sin is a disordered act which has an effect on the sinner.
It turns our hearts away from God, and toward some created good instead. The sinner needs purification from these disordered inclinations. More than a punishment, this purification is a natural consequence of the evil of sin. There are two kinds of indulgences, plenary, which removes all the temporal punishment due, and partial, which shortens or lessens that punishment. A plenary indulgence can be acquired only once a day, unless at the point of death.