Indulgences are, in the Church, a true spiritual treasure laid open to all the faithful; all are permitted to draw therefrom, to pay their own debts and those of others.
Does access to this treasury demand painful efforts on our parts… Do we not see how men for love of gold, in order to preserve a work of art, to save part of their fortune or a precious fabric, expose themselves to the flames of a fire? Ought we not then to do at least as much to save from expiatory flames those souls ransomed by the Blood of Jesus Christ? But Divine goodness asks nothing so painful: it requires only such works as are ordinary and easy—a Rosary, a Communion, a visit to the Blessed Sacrament, an alms or the teaching of the elements of the Catechism to abandoned children.
—from Purgatory: Explained by the Lives and Legends of the Saints by Fr. F.X. Schouppe, S.J., Part Second: Purgatory, The Mystery of God’s Mercy – Ch. 26
BONUS: Here is an excellent Primer on Indulgences from Catholic Answers which explains the Biblical principles behind the Church’s infallible teaching on indulgences.