Please read A Reflection on the Sacrament of Confession.
Jesus entrusted the ministry of reconciliation to the Church. The Sacrament of Penance is God's gift to us so that any sin committed after Baptism can be forgiven. In confession we have the opportunity to repent and recover the grace of friendship with God. It is a holy moment in which we place ourselves in his presence and honestly acknowledge our sins, especially mortal sins. With absolution, we are reconciled to God and the Church.
The Sacrament helps us stay close to the truth that we cannot live without God. "In him we live and move and have our being" (Acts 17:28). While all the Sacraments bring us an experience of the mercy that comes from Christ's dying and rising, it is the Sacrament of Reconciliation that is the unique Sacrament of mercy. - From the
United States Catholic Catechism for Adults.