No. 1468, 1469 and 1470, page 369 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:
This sacrament reconciles us with the Church. Sin damages or even breaks fraternal communion. The sacrament of Penance repairs or restores it. In this sense it does not simply heal the one restored to ecclesial communion, but has also a revitalizing effect on the life of the Church which suffered from the sin of one of her members. (Cf. 1 Corinthians 12: 26). Re-established or strengthened in the communion of saints, the sinner is made stronger by the exchange of spiritual goods among all the living members of the Body of Christ, whether still on pilgrimage or already in the Heavenly homeland:(Cf. Lumen Gentium 48-50)
In this sacrament, the sinner, placing himself before the merciful judgment of God, anticipates in a certain way the judgment to which he will be subjected at the end of his earthly life. For it is now, in this life, that we are offered the choice between life and death, and it is only by the road of conversion that we can enter the Kingdom, from which one is exculded by grave sin. (Cf. 1 Corinthians 5: 11; Galatians 5: 19-21; Revelation/Apocalypse 22: 15) In converting to Christ through penance and faith, the sinner passes from death to life and "does not come into judgment." (John 5: 24).
From the Baltimore Catechism No. 3; Benziger Brothers, Inc. and Tan Books and Publishers, Inc. Nos. 803, 804 and 805, page 169.
Q. Does not the Sacrament of Penance remit all punishment due to sin?
A. The Sacrament of Penance remits the eternal punishment due to sin, but it does not always remit the temporal punishment which God requires as satisfaction for our sins.
Q. Why does God require a temporal punishment as a satisfacation for sins?
A. God requires a temporal punishment as a satisfaction for sin to teach us the great evil of sin and to prevent us from falling again.
Q. Which iare the chief means by which we satisfy God for the temporal punishment due to sin?
A. The chief means by which we satisfy God for the temporal punishment due to sin are: Prayer, Fasting, Almsgiving; all spiritual and corporal works of mercy, and the patient suffering of the ills of life.