Catechism of the Catholic Church §2856 — excerpt
"Then, after the prayer is over you say 'Amen,' which means 'So be it,' thus ratifying with our 'Amen' what is contained in the prayer that God has taught us."
This translation is under copyright, so only the cited words are quoted here. Follow the link for the full text. Translation: English translation of the Catechism of the Catholic Church, 2nd ed. (Libreria Editrice Vaticana), as posted at vatican.va.
Also cites: St. Cyril of Jerusalem, Catech. myst. 5,18: PG 33, 1124