I have heard Catholics speak of indulgences for the souls in purgatory? What are indulgences?

The Catholic Answers

Do not mix up the ecclesiastical term indulgence with the modern idea of self-indulgence. An indulgence is not a permission to indulge in sin, but is a remission of punishment due to sin. Now in the early Christian Church certain sins were punished by long public penance, sometimes for days, at other times for years. But the Church was often indulgent, and loosed or freed Christians from all or part of their public penance, if they showed other good dispositions, or performed certain works of charity. The Church had that power in the name of God as surely as the state has the power in its own name to commute a sentence or even release a criminal altogether under certain circumstances. Christ said to the Church, “Whatsoever you shall loose upon earth shall be loosed also in heaven.” Matt. XVIII., 18. That the merits of Christ and of the Martyrs and Saints of the ages are at the disposal of the Church is also a consequence of the doctrine of the Communion of the Saints. And that power of commuting or even of remitting penances and expiations exists in the Church to-day, being exercised by the granting of indulgences.

Radio Replies Volume 1 by Rev. Dr. Leslie Rumble MSC and Rev. Charles Mortimer Carty