The Sacred Liturgy Nos. 7 & 8

7. The Act of Penitence…
At the beginning of the Mass, after making the Sign of the Cross, and greeting the people, the priest invites the people to call to mind their sins in preparation for the celebration of the Mass. Silence is then observed, so that everyone present may recollect themselves and call to mind the times when they have not fulfilled the commandments of the Lord. We humbly acknowledge that we are imperfect, and that we have sinned, before we approach the altar to celebrate the Mass. The priest prays that the Lord will have mercy on us and forgive us our sins. The Act of Penitence always ends in the Kyrie Eleison, Greek for “Lord, have mercy.” By this prayer, we implore the Lord to have mercy on us.
It is important to remember that the Act of Penitence is a prayer for mercy and forgiveness. It does not absolve sins in the same way as the Sacrament of Reconciliation. If a person is conscious of grave sin, he must go to Confession before receiving Communion.
GIRM 51-52

8. The Structure of the Act of Penitence…
The rubrics of the Mass [rubric means “liturgical directive” and is derived from the Latin rubrico, which means “red,” because the rubrics are printed in red in the liturgical books] offer the Celebrant three options for the Act of Penitence.
A. The recitation of the Confiteor is the normative option. It is a prayer said by the whole assembly, in which we confess our sinfulness and beg God’s mercy.
B. The second option is a series of invocations, and responses by the people, which express our contrition and prayer for mercy.
C. The third option is the “Kyrie” with invocations preceding each verse. (“Lord, have mercy,” is from the Greek Kyrie, eleison.) For example: “You were sent to heal the contrite: Lord, have mercy.”
All three of these are preceded by the priest’s invitation and followed by the absolution prayer (“May almighty God have mercy on us, forgive us our sins, and bring us to everlasting life.”) Options A and B do not include the “Kyrie,” so it is recited after the absolution prayer when these options are used.
GIRM 51-52