To understand the Church’s teaching on indulgences one must understand the difference between the eternal punishment due sin and the temporal punishment due sin.
**Catechism of the Catholic Church
The punishments of sin
1472** To understand this doctrine and practice of the Church, it is necessary to understand that sin has a double consequence. Grave sin deprives us of communion with God and therefore makes us incapable of eternal life, the privation of which is called the “eternal punishment” of sin. On the other hand every sin, even venial, entails an unhealthy attachment to creatures, which must be purified either here on earth, or after death in the state called Purgatory. This purification frees one from what is called the “temporal punishment” of sin. These two punishments must not be conceived of as a kind of vengeance inflicted by God from without, but as following from the very nature of sin. A conversion which proceeds from a fervent charity can attain the complete purification of the sinner in such a way that no punishment would remain.
**1473 ** The forgiveness of sin and restoration of communion with God entail the remission of the eternal punishment of sin, but temporal punishment of sin remains. While patiently bearing sufferings and trials of all kinds and, when the day comes, serenely facing death, the Christian must strive to accept this temporal punishment of sin as a grace. He should strive by works of mercy and charity, as well as by prayer and the various practices of penance, to put off completely the “old man” and to put on the “new man.”
… if a person commits the sin of backbiting or slander, what is the temporal punishment for that
The temporal punishment of sin “must not be conceived of as a kind of vengeance inflicted by God”. A person that commits the sin of backbiting or slander shows that his love is still imperfect. That person must perfect his love before he can enter Heaven. He will either perfect his love on earth, or he will do it in Purgatory. To perfect his love on earth, he should “strive by works of mercy and charity, as well as by prayer and the various practices of penance, to put off completely the “old man” and to put on the “new man.”