The doctrine and practice of indulgences in the Church are closely linked to the effects of the sacrament of Penance.
"An indulgence is a remission before God of the temporal punishment due to sins whose guilt has already been forgiven, which the faithful Christian who is duly disposed gains under certain prescribed conditions through the action of the Church which, as the minister of redemption, dispenses and applies with authority the treasury of the satisfactions of Christ and the saints."
"An indulgence is partial or plenary according as it removes either part or all of the temporal punishment due to sin." Indulgences may be applied to the living or the dead.
Source: Catechism of the Catholic Church (Vatican.va)