Jesus died on the Cross and rose from
the dead to reconcile sinful people with God through the forgiveness of sins and the gift of new life with the Triune God. Even on the Cross, He forgave those who were killing Him and had mercy on the repentant thief. Only God can forgive our sins. But Jesus willed that the Church should be His instrument of forgiveness on earth.  

In the Sacrament of Reconciliation, the priest acts in the person of Christ, the Head of the Church, to reconcile the sinner to both God and the Church. "When he celebrates the Sacrament of Penance, the priest is fulfilling the ministry of the Good Shepherd who seeks the lost sheep.... The priest is the sign and instrument and instrument of God's merciful love for the sinner" (CCC, no. 1465).

The Sacrament of Penance involves a conversion of our hearts to God, a confession of sins to a priest, the forgiveness of our sins, a penance to make some amends for sin, and reconciliation with God and the Church. For those who commit mortal sin after Baptism, this Sacrament is necessary for being reconciled to God and the Church.  

Confession liberates us from sins that trouble our hearts and makes it possible to be reconciled to God and others. We are asked to look into our souls and, with an honest and unblinking gaze, identify our sins. This opens our minds and hearts to God, moves us toward communion with the Church, and offers us a new future. 

When we have examined our consciences and have taken responsibility for our sins, we then confess them to the priest. By naming our sins before the priest, who represents Christ, we face our failings more honestly and accept responsibility for our sins.  We must confess all our mortal sins in kind and number. After we confess our sins, we are given some encouragement from the priest for our moral and spiritual growth. The priest then gives us a penance and asks us to say an Act of Contrition. Then the priest grants absolution, that is, he sets us free from our sins, using the power that Christ entrusted to the Church and by which he pardons the sins of the penitent (cf. CCC, no. 1424). 

"Absolution takes away sin, but does not remedy all the disorders sin has caused" (CCC, no. 1459). It is obvious that we need to repair certain damages that our sins have caused, such as restoring the reputation of someone we have injured, returning money that we have stolen, or rectifying an injustice. Sin also weakens the relationship we have with God and others. Our inner life is harmed by sin and needs restoration. This is the reason for acts of penance and satisfaction for our sins. The penance given by the priest helps us to begin making satisfaction for our sins. Just as when we get physically out of shape, we need to take up some exercise, so also when the soul is morally out of shape, there is the challenege to adopt spiritual exercises that will restore it.  

The reception of this sacrament ought to be prepared for by an examination of conscience made in the light of the Word of God. The passages best suited to this can be found in the Ten Commandments, the moral catechesis of the Gospels and apostolic Letter, such as the Sermon on the Mount and apostolic teaching. (CCC, no. 1454)

The Church declares that every priest who hears confessions is bound under very severe penalties to keep absolute secrecy regarding the sins that his penitents have confessed to him. He can make no use of knowledge that which admits of no exceptions, is called the "sacramental seal," because what the penitent has made known to the priest remains "sealed" by the sacrament. (CCC, no. 1467) 

The Sacrament of Penance is an experience of the gift of God's boundless mercy. Not only does it free us from our sins but it also challenges us to have the same kind of compassion and forgiveness for those who sin against us. We are liberated to be forgivers. 

-taken from the United States Catholic Catechism for Adults

An Act of Contrition
O my God, I am heartily sorry for having offended thee and I detest all my sins, because of thy just punishments, but most of all because they offend thee, my God, who are all good and worthy of all my love. I firmly resolve, with the help of thy grace, to sin no more and to avoid the near occasions of sin. Amen.