"An indulgence does not confer grace. An indulgence is not a remission of the guilt due to sin. The guilt due to sin is ordinarily taken away by the Sacraments of Baptism and Penance (confession), in which we receive forgiveness for sins through Jesus Christ," the United States Conference of Catholic Bishops (USCCB) explains in this helpful Q&A.
"Although guilt is taken away, and with it the eternal penalty that is due to sin, namely, damnation, the eternal loss of the presence of God, there remain consequences for sins that those who have committed them must bear. There is what is traditionally called the temporal punishment for sin."
What is temporal punishment? As the USCCB explains, "Every sinful act creates a disorder within the soul of the human person; it distorts our desires and affections, leaving us with 'an unhealthy attachment to creatures, which must be purified either here on earth, or after death in the state called Purgatory.'"
For this reason, the USCCB continues, "Those who have received forgiveness for their sins still have an obligation to undergo a difficult and painful process (the temporal penalty for sin) to be purified of the consequences of their sins and to restore the disrupted relationships."
Through a plenary indulgence, we can be spared this purification process. As the USCCB explains it, "By God's grace, participation in a prayer or action that has an indulgence attached to it brings about the necessary restoration and reparation without the suffering that would normally accompany it."
On Holy Thursday, you can receive this special gift, resulting in the purification of your soul, by singing the "Tantum Ergo." If you aren't familiar with this beautiful hymn, watch this short video.