1. A plenary indulgence is a grace granted by the Catholic Church through the merits of Jesus Christ to remove the temporal punishment due to sin. It cleanses the soul as if the person has just been baptized.
2. The Holy Father declares that the gift of the Indulgence "is a way of discovering the unlimited nature of God’s mercy. Not by chance, for the ancients, the terms ‘mercy’ and ‘indulgence’ were interchangeable, as expressions of the fullness of God’s forgiveness, which knows no bounds" (Spes non confundit, 23). The Indulgence, therefore, is a Jubilee grace.
3. You can use this Indulgence for souls in purgatory. A plenary indulgence obtained for them would obtain for them the release from purgation and the entrance into heavenly glory.
4. To obtain either a partial or plenary indulgence, a person must be baptized, in the state of grace (at least at the end of the spiritual work to be done) and not excommunicated, and must have the general intention to actually obtain the indulgence itself.