Catechism of the Catholic Church §1062 — excerpt
In Hebrew, amen comes from the same root as the word "believe." This root expresses solidity, trustworthiness, faithfulness. and so we can understand why "Amen" may express both God's faithfulness towards us and our trust in him.
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.