It is called the sacrament of confession, since the disclosure or confession of sins to a priest is an essential element of this sacrament. In a profound sense it is also a "confession" - acknowledgment and praise - of the holiness of God and of his mercy toward sinful man.
It is called the sacrament of forgiveness, since by the priest's sacramental absolution God grants the penitent "pardon and peace."
It is called the sacrament of Reconciliation, because it imparts to the sinner the love of God who reconciles: "Be reconciled to God." He who lives by God's merciful love is ready to respond to the Lord's call: "Go; first be reconciled to your brother."
Source: Catechism of the Catholic Church (Vatican.va)