Jesus included forgivenesss of our trespases in the Our Father prayer. Unfortunately, sin is part of our human record of behaviour on earth. Jesus in His going around the Holy Land forgave sinners in addition to healing the sick. And before He returned to heaven, He conferred on the Apostles the power to forgive sins in His first post-resurrection appearance to them.
The Sacrament of Reconciliation embodies this power to forgive sins. It is administered by the Church through the bishops who in turn grant this faculty to their priests.
So when the confessor pronounces the final words of the formula of absolution: “and I absolve you from your sins in the name of the Father, and of the Son, and of the Holy Spirit””and I absolve.”he is following the directive of Jesus when He instituted the sacrament. Of course, the forgiveness depends on the power of God and not the confessor even though he says “and I absolve…”
The one who confesses must be contrite with a sorrow of the soul and detestation for the sin committed, together with the resolution not to sin again. If the contrition arises from the love of God above all, then it is called perfect contrition. If the sorrow arises from a lesser motive then it is called imperfect contrition.
The penitent has to tell all mortal sins, the serious offences that sever the sanctifying grace link with God. And this has to be done at least once a year if there is a mortal sin and also whenever there is a mortal before one can receive Communion..
The confessor concludes the confession by imposing a penance, usually some prayers, which the penitent has to accept.
So in summary, the Sacrament of Reconciliation consists in three actions of the penitent and the priest’s absolution. The penitent’s acts are repentance, confession or disclosure of of sins to the priest, and the intention to perform the penance assigned by the priest.
Every priest who hears confessions is bound under very severe penalties to keep absolute secrecy regarding the sins confessed and can never use that information in any way..