Indulgentia est remissio coram Deo poenae temporalis pro peccatis, ad culpam quod attinet iam deletis.
– From Indulgentiarum doctrina, norm 1
An indulgence is the remission of temporal punishment due to sins which have already been forgiven. The Church offers to the faithful the opportunity to gain both plenary and partial indulgences (i.e., those which free the person from all or from some of the temporal punishment due to sin), for either themselves or for the dead. The Christian who is “rightly disposed and [observes] the definite, prescribed conditions” receives the indulgence, through the Church, from the “treasury of the expiatory works of Christ and His saints” (Indulgentiarum doctrina norm 1 [hereafter, ID 1]).
After we die, our souls will immediately undergo the particular judgment, and the soul will then proceed to its punishment or its reward (BC 1373, CCC 1022). Those souls that, while assured of their salvation, have not satisfied punishment due to their sins will proceed to Purgatory, the temporary state of purification they must endure before the Beatific Vision. (BC 1383, CCC 1030). Indulgences can partially or fully satisfy the temporal punishment due to sins for either ourselves or for the faithful departed (BC 853, BC 1385, CCC 1032, ID 2).
Indulgences are thus a great gift to the faithful, and they offer us an opportunity to humbly seek God’s mercy for ourselves and the faithful departed.
To receive an plenary indulgence (which may be gained once per day, except at the hour of death), the member of the faithful must do the prescribed work, be baptized and in a state of grace, not excommunicated, free from all attachment to sin (even venial sin), and fulfill the following conditions: sacramental confession, Eucharistic communion, and prayer for the pope’s intentions (ID 20-23).
Indulgences ought to be an important part of any Catholic’s spiritual life. The penance and prayer done through them is magnified by the Church’s treasury of merit, through the merits of Christ. Seeking to receive indulgences also reminds and encourages us to go to Mass and confession regularly, and seek always to be free from attachment to sin. They also expose the faithful to various acts of piety which they may otherwise have been unaware of.
I recommend The Handbook of Indulgences: Norms and Grants, the authorized English edition published by the Catholic Book Publishing Corporation. The book provides a rich source for traditional prayers and works of piety, and contain the norms for receiving the indulgences attached to them.