Explain the purpose of the sale of Indulgences.

Indulgences were a theological principle of forgiveness and punishment in the Catholic Church.
In its original form, an Indulgence was a way of escaping punishment in the afterlife for one's sins. It was not, as the name might otherwise suggest, a way of granting forgiveness or freedom from guilt. To the Catholics, that forgiveness could only be granted by God via the sacrament of Confession. However, even if the guilt of the sin had been forgiven, the sin still required temporal punishment. This punishment could be served as time in purgatory in the afterlife--or it could be reduced in the real world. Indulgences--payments, prayers, and other forms of physical contrition--were a way to reduce that debt from one's soul and escape physical punishment or a lengthy confinement to purgatory before admission to heaven.
Unfortunately, the principle of Indulgences became thoroughly corrupted. By the late Middle Ages, many church officers and clerics were using the practice to enrich themselves by selling forgiveness for extravagant sins for extravagant sums of money. This corruption was one of the main motivators for the Protestant movement's split from the Catholic Church.
https://www.britannica.com/topic/indulgence