What is the medieval Christian concept of “indulgence” often linked to crusading?

The story behind the answer

An indulgence, in medieval theology, remitted the temporal penalties due for sin after the sinner’s guilt had been absolved in confession. It did not erase the crime itself, guarantee heaven, or allow people to purchase salvation; rather, it addressed the remaining penance or time in purgatory.

Linked to crusading, popes offered participants a plenary indulgence—full remission of these temporal penalties—most famously at the First Crusade when Urban II promised “remission” to those who took the cross. The rationale drew on the Church’s power to bind and loose and the “treasury of merit” of Christ and the saints, applied to the faithful who met conditions like contrition, confession, and charitable acts. Although later abuses (such as selling indulgences) provoked controversy and fueled the Reformation, the original idea was a spiritual benefit tied to sincere repentance and strenuous works, including the extreme sacrifice of a crusade.