An indulgence is a pious act ordered by the church which you must complete after being forgiven from a temporal sin in confession. There are many different acts that could be given some of which are praying or reading passages in the bible for a certain amount of time. Without completing the indulgence your forgiven confession will not be valid and when you die you will go to Purgatory where you will have to complete the uncompleted indulgence before going on to heaven. Some believe that you can complete your indulgence with money, but now that is not at all the case.