In Catholic teaching the nature or essence of the Sacrament of Penance is composed of “matter” and “form”. These are the essential parts of the Sacrament. For this reason, there is no Sacrament if either the matter or the form is absent.
The Church teaches that the “matter” of the Sacrament of Penance is “the three acts of the penitent”. These acts include contrition, confession and satisfaction. In the first place, the first act of the penitent, “contrition,” refers to the “sorrow of the soul and detestation for the sin, together with the resolution not to sin again” (CCC 1451). This can either be perfect or imperfect contrition. Perfect contrition in a penitent proceeds from his true love of God by loving God as his First Love. He can also develop imperfect contrition. This form of contrition is born of the ugliness of sin and the fear of eternal damnation. Furthermore, the second act of the penitent is the “confession” of his sins to a priest (CCC 1456, 1458). On the one hand, the Church “recommends” that the penitent confess his venial sins. These are sins that spiritually “wound” his life of charity interiorly. On the other hand, the Church “requires” that the penitent confess all his particular mortal sins. These are sins that “destroy” his spiritual life of charity. In this sense, he suffers spiritual death by mortal sin. Finally, the third act of the penitent is “satisfaction” for sin (CCC 1459). Through satisfaction he offers reparation to heal the damage he has done to himself, his relationship with God, the Church and his neighbor. This is the “penance” the priest assigns to the penitent in this Sacrament. For instance, this would include returning the goods he has stolen from his neighbor. This would also mean restoring the good name or reputation of a person he may have slandered.
The “form” of Penance is the prescribed “words of absolution,” which the priest pronounces in absolving the penitent of his sins. This is the priest acting in the person of Christ the Head to forgive sins through the sacred power he has received from God through his ordination. In doing so, Christ acts through the agency of the priest (CCC 1449).