What does the word indulgence mean? It comes from the Latin word Indulgentia, which came from Indulgeo (to be kind or tender like the kindness and mercy of God). Later it came to mean remission of a tax or debt.
An indulgence is the remission of the temporal punishment due to sin, the guilt of which has been already forgiven by a sincere confession. The Church has made ways for the penitent to make satisfaction for his debts to God by tapping into the treasury of merits of Christ and the saints. The Church, who has been given the Keys by Jesus, mitigates punishment incurred (temporal penalties) by opening and applying the treasury of merits Christ and the saints have gained by their sacrifices and applies these merits to the faithful under certain conditions.
What is a plenary indulgence? When you go to confession the guilt of your sins are removed and also the eternal punishment due to mortal sin. But there still remains the temporal punishment required by Divine Justice (for rebelling against and offending God by sinning). So a plenary indulgence is the complete paying (by the mercy of God and from the merits of all the saints) of the all temporal punishment required by the Divine justice for all mortal sins (up to that time) that have been already confessed well.
It is reparation for the damage done to God, the Church, society, others and oneself. It is making up for and repairing the damage done by a mortal sin already confessed and the penance completed.
Why do we need a plenary indulgence? Often we go to confession and do the penance, but never do (or are unable to do) the reparation of the damage done by our sins to God, ourselves and others. So by certain prayers and actions the Church acting, in the place of God, grants plenary and partial indulgences.
How do we gain a plenary indulgence? First the person who wants the plenary indulgence has to be free of mortal sin and has to complete what is asked of by the Church.
In the case of the plenary indulgence given during the beginning of November (Nov 1-8), one must go and pray at a cemetery. Then the other normal requirements must be fulfilled too. Going to Holy Mass, Receiving Holy Communion, Going to confession, and a Our Father, a Hail Mary and Glory be for the pope’s intention with in 8 days. With these done, one can receive a plenary indulgence for someone in purgatory (this plenary indulgence on November can only be applied to a soul in purgatory). Others can be applied to oneself or for a soul in purgatory.
What does a Plenary Indulgence do for a soul in purgatory? Once the prayers and actions have been completed, the soul for whom the plenary indulgence was done for, gets instantly out of purgatory.
Also on November 2nd. a plenary indulgence can be gained for a soul in purgatory by those who visit a church or chapel on All Soul’s Day with the prayers of 1 Our Father and the Creed. Only on this day you can repeat this as often as you want to gain more plenary indulgences for more souls. All other days only one plenary indulgence can be gained. I would assume that you need to go to confession, Holy Mass and receive Holy Communion once within 8 days.