Indulgences, under the doctrine of the Roman Catholic Church, are specific recommended prayers or actions which are intended to help bring a person, dead or alive, closer to perfection. A result of this is the lessening of the burning need to bring that person to a perfect state, necessary to enter Heaven. A person may seek an indulgence for a loved one who has passed away or for the temporal punishment of oneself. A plenary indulgence may be obtained for the living or the dead, known or unknown. While Sacramental Confession is needed for the forgiveness of sins, indulgences bring the sinner closer to perfection in his very person, thus reducing time spent in the state known as Purgatory. An indulgence cannot be seen as a replacement for confession, for obvious reasons.
Until the 1567 Council of Trent, indulgences could be bought for specific periods of time. However, when these old-form indulgences used time-based terms, they didn't mean "x number of days off Purgatory." The time-based term meant that the indulgence remitted the same amount of temporal punishment as would that much time spent in penance on earth. Thus, an indulgence of 300 days meant it remitted the temporal punishment that 300 days of penance would.