An indulgence is the full or partial remission of temporal punishment for sins after the sinner confesses and receives absolution.  Under Catholic teaching, every sin must be purified either here on earth or after death in a state called purgatory. . In Catholic theology, such indulgences eliminate the time in which souls are cleansed in purgatory before entry into heaven
The 6th century saw the development in Ireland of Penitentials, handbooks for confessors in assigning penance. It became customary to commute penances by the payment of fixed sums of money depending on the various kinds of offenses (tariff penances). 
Some commissaries sought to extract the maximum amount of money for each indulgence. Professional pardoners practiced the unrestricted sale of indulgences. Many of these pardoners exceeded official church doctrine, and promised rewards such as salvation from eternal damnation in return for money.
Currently you cannot buy one — the church outlawed the sale of indulgences in 1567 — but charitable contributions, combined with other acts, can help you earn one. There is a limit of one plenary indulgence per sinner per day.