In typical dogmatic
obfuscation: "the remission before God of the temporal punishment due for sins already forgiven as far as their guilt is concerned"
This is a strange area of Catholic dogma where in addition to being forgiven for your sins by confessing, you are "cleansed" of the damage that the sin has done to you. They never covered this during my indoctrination, but apprently there are some earthly punishments imposed by God when you sin in addition to the eternal punishment that supposedly awaits you after death. It's kind of a way to short circuit the idea that what goes around comes around.
To recieve one, you must perform one of a series of eligible acts, confess, take communion, and pray for the pope.
Partial list of eligible acts:
Reading of Sacred Scripture (at least a half hour)
Recitation of the Marian Rosary (in a group for full indulgence)
Exercise of the Way of the Cross (aka , Stations of the Cross)
Receiving a Papal Blessing (including by tv & radio)
Sources
http://www.tomlinson.org/plenary.htm
http://www.ewtn.com/library/ANSWERS/PRIMINDU.HTM