What is happening here
Before the Liturgy of the Word and the Eucharist, the Church pauses for a brief acknowledgment of our need for God's mercy. The Penitential Act takes one of three forms in the current Roman Rite: the Confiteor ("I confess to almighty God..."), a shorter dialogue, or an invocation form known as the Kyrie ("Lord, have mercy. Christ, have mercy. Lord, have mercy.").
The Penitential Act is not the sacrament of confession. It does not absolve mortal sin — Catholic teaching is clear that grave sin requires sacramental reconciliation before approaching the Eucharist. What the Penitential Act does is gather the assembly's general consciousness of sinfulness and place it before God's mercy at the start of the liturgy. It prepares the heart to receive the Word and the Sacrament with appropriate humility.
The Confiteor includes the striking phrase "through my fault, through my fault, through my most grievous fault" — a deliberately strong acknowledgment that sin is real, my own, and serious. The threefold "Lord, have mercy" of the Kyrie is one of the oldest Christian prayers, used in the Eastern liturgies for centuries before it was incorporated into the Roman Mass. To pray it is to join one's voice to a chorus that has been crying for mercy across nations and across centuries.
The act concludes with the priest's prayer of absolution: "May almighty God have mercy on us, forgive us our sins, and bring us to everlasting life." It is a general intercession, not a sacramental one — but the mercy invoked is real, and the heart that receives it is being prepared for what comes next.