During the Holy Year, pilgrims may gain a plenary indulgence through any of the following actions:
In addition, to gain the plenary indulgence, one must:
If any of these conditions are not met, an indulgence is partial. Only one plenary indulgence may be obtained in a day, either for oneself or a departed soul. Multiple partial indulgences may be obtained in a day.
Latin text of the Indulgence Decree
Indulgences are the remission before God of the temporal punishment due to sins whose guilt has already been forgiven. The faithful Christian who is duly disposed gains the indulgence under prescribed conditions for either himself or the departed. Indulgences are granted through the ministry of the Church which, as the dispenser of the grace of redemption, distributes the treasury of the merits of Christ and the Saints. (Catechism 1498)
Our sins have two results:
In this way, indulgences remit/reduce the temporal punishment of sin. But in addition to my own penances, I can receive help to purify my heart from other members of the Church, the Body of Christ. In the Body of Christ, all the members are linked to each other through love. Thus, the prayers and sacrifices of one person can benefit others. The indulgence give us the spiritual help of the saints, the Blessed Virgin and above all, Christ himself, so that our hearts can be more quickly purified.
All the prayers and sacrifices done by Christians in union with Christ are considered a kind of spiritual treasury, which can benefit not only oneself but others in the Church. Through an indulgence, the Church helps the individual Christian from this "treasury".
A plenary indulgence remits all the temporal punishment due to sin. This is why one of the conditions is no attachment to venial sin. If the heart is attached to sin, it cannot be purified (i.e. love Christ above all else). A partial indulgence remits some of the temporal punishment due to sin.