An indulgence is a way to reduce the amount of punishment one has to undergo for sins. An indulgence may reduce either or both of the Penance required after a sin has been forgiven, or the time in Purgatory. The recipient must perform certain requirements to receive the indulgence. Refer to the article Jubilee of Mercy for an instance of requirements. The Jubilee of Mercy is a Plenary Indulgence which may erase the total punishment for sins committed till the indulgence, whereas a Partial Indulgence only reduces the punishment.
“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.” (Rule 1 of the Indulgences Manual)
Particularly important is the plenary indulgence that a priest imparts when giving the sacraments to a person in danger of death; if no priest is available, the Church grants the plenary indulgence to any rightly disposed Christian at the moment of death. Refer to the article Anointing of the Sick.
A plenary indulgence is normally authorized to those properly disposed who attend a newly ordained priest’s First Mass. This is not the same as any indulgence granted from attending the Ordination Mass itself, since the ordaining Bishop is the principal celebrant.