What is a plenary indulgence?
Confession removes sin. A plenary (meaning full) indulgence frees one from the temporal consequences/punishment of sin. Even after we’ve been forgiven, we may still experience the consequences of sin. The Church steps in by using the keys that Jesus gave it to remove the guilt of the sin.
Take for example a student in English class who intentionally copies someone else’s work. The teacher responds: “I forgive you, but you will stay after school three days to write your own essay.” The student’s actions are forgiven, but the action has consequences, and the student must learn his lesson.
The Church intercedes with a plenary indulgence where the person must complete the indulgence act (in this case, visiting the relics), go to confession and communion (within ten days on either side of the indulgence), and pray for the intentions of the Pope. When we do these things and truly show we have taken to heart what we have done, a plenary indulgence is granted.