No. 1474, 1475, 1476, 1477 and 1498, page 371and page 374 of the Catechism of the Catholic Church: The Celebration of the Christian Mystery; Chapter Two - The Sacraments of Healing; Libreria Editrice Vaticana: Urbi Et Orbi Communications:
The Christian who seeks to purify himself of his sin and to become holy with the help of God's grace is not alone. "The life of each of God's children is joined in Christ in a wonderful way to the life of all the other Christian barethren in the supernatural unity of the Mystical Body of Christ, as in a single mystical person." [Indulgentiarum doctrina, 5].
In the communion of saints, "a perennial link of charity exists between the faithful who have already reached their Heavenly home, those who are expiating their sins in Purgatory, and those who are still pilgrims on earth. Between them there is, too, an abundant exchange of all good things." [ibid.] In this wonderful exchange, the holiness of one profits others, well beyond the harm that the sin of one could cause others. Thus recourse to the communion of saints lets the contrite sinner be more promptly and efficaciously purified of the punishments for sin.
We also call these spiritual goods of the communion of saints the Church's treasury, which is "not the sum total of the material goods which have accumulated during the course of the centuries. On the contrary the 'treasury of the Church' is the infinite value, which can never be exhausted, which Christ's merits have before God. They were offered so that the whole of mankind could be set free from sin and attain communion with the Father. In Christ, the Redeemer Himself, the satisfactions and merits of His Redemption exist and find their efficacy." [Ibid.]
"This treasury includes as well the prayers and good works of the Blessed Virgin Mary. They are truly immense, unfathomable, and even pristine in their value before God. In the treasury, too, are the prayers and good works of all the saints, all those who have followed in the footsteps of Christ the Lord and by His grace have made their lives holy and carried out the mission the Father entrusted to them. In this way they attained their own salvation and at the same time cooperated in saving their brothers in the unity of the Mystical Body." [Ibid.]
Through indulgences the faithful can obtain the remission of temporal punishment resulting from sin for themselves and also for the souls in Purgatory.