When we commit a sin and repent of the sin, we are forgiven by God but there is still a penalty to paid for the sin. For example, you can break someone's window and they can forgive you, but you still have to pay for a new window to replace the one you broke.

An indulgence is a remission of penalty due to sin. The greater the sin, the greater the penalty.

There are two times when we can remit our sins: in this life, or in Purgatory. Since you cannot pay for your sins while in Purgatory, it can only be done now. You can either pray for yourself, to remit your own sins, or better still, pray for those who are in Purgatory and help them first.

If you would like to learn more about indulgences, take a look at The Enchiridion of Indulgences web site.