The Hebrew word “Amen”, which also concludes the last book of Sacred Scripture, some of the prayers of the New Testament, and the liturgical prayers of the Church, expresses our confidence and total “yes” to what we professed in the Creed, entrusting ourselves completely to him who is the definitive “Amen” (Revelation 3:14), Christ the Lord. It is spoken to express solemn ratification or agreement.
When one says “amen” in response to a prayer, it serves as an affirmation of agreement with the content of the prayer (cf. 1 Cor 14:16) – in which case it is sometimes translated as “So be it” (cf. CCC 2856)—or as an expression of faith that God will hear and act on the prayer.