Food for Thought
What is an Indulgence?
Every sin has consequences. It disrupts our communion with God and the Church, weakens our ability to resist temptation, and hurts others. The necessity of healing these consequences, once the sin itself has been forgiven, is called temporal punishment. Prayer, fasting, almsgiving, and other works of charity can take away entirely or diminish this temporal punishment. Because of the fullness of redemption obtained for us by Christ, the Church attaches to certain prayers and actions an indulgence or pardon, that is, the full or partial remission of temporal punishment due to sin. Christ, acting through the Church, brings about the healing of the consequences of sin when an individual uses such a prayer or engages in such an action
(The US Catholic Catechism for Adults, p. 244).
Further reading: CCC 1471-1479