It is buying with works or cash some of the excess merits of Christ and of Mary and the saints.. stored in that Catholic treasury .
The full merit of Christ was applied at the cross.. that is why there is no need for man to purge himself..
Ohhhh, I'm sorry, that's not the answer we were looking for. But we do have a lovely parting gift for you.
"An indulgence is obtained through the Church who, by virtue of the power of binding and loosing granted her by Christ Jesus, intervenes in favor of individual Christians and opens for them the treasury of the merits of Christ and the saints to obtain from the Father of mercies the remission of the temporal punishment due for their sins." The Church does this not just to aid Christians, "but also to spur them to works of devotion, penance, and charity" (CCC 1478).
The financial scandal surrounding indulgences, the scandal that gave Martin Luther an excuse for his heterodoxy, involved alms—indulgences in which the giving of alms to some charitable fund or foundation was used as the occasion to grant the indulgence. Contrary to Reformist propaganda there was no outright selling of indulgences.
The Council of Trent instituted severe reforms in the practice of granting indulgences, and, because of prior abuses, "in 1567 Pope Pius V canceled all grants of indulgences involving any fees or other financial transactions" (Catholic Encyclopedia). This act proved the Church’s seriousness about removing even an opportunity for abuses from indulgences.