From the U.S. Conference of Catholic Bishops:
The Sacrament of Penance must be seen within the context of conversion from sin and a turn to God. Peter wept bitterly over his triple denial of Christ but received the grace of conversion and expressed it with a threefold confession of love for Jesus (cf. Lk 22:54-62; Jn 21:15-19). Paul was converted from persecuting Christians to becoming one of the greatest disciples of Christ who ever lived (cf. Acts 9:1-31). These moments of conversion were only the beginning of their lifelong commitment to living in fidelity to the Gospel of Jesus Christ.
Sin harms our relationship with God and damages our communion with the Church. Conversion of heart is the beginning of our journey back to God. Liturgically this happens in the Sacrament of Penance. In the history of the Church, this Sacrament has been celebrated in different ways. Beneath the changes, there have always been two essentials: the acts of the penitent and the acts of Christ through the ministry of the Church. Both go hand in hand. Conversion must involve a change of heart as well as a change of actions. Neither is possible without God’s grace.
Resources:
Penance: Reconciled to Right Relationship, Called to Heal and Restore (Also available en Espanol) – This short, two-page handout can assist your reflection on this sacrament as an opportunity to reestablish right relationship with God and others, and to recognize your participation in the “structures of sin” which degrade others’ lives and dignity.