I must say that word a hundred times a day: over and over again in the Rosary, whenever I make the sign of the cross, after every prayer, and when I receive Our Lord in the Blessed Sacrament. I hardly ever think about what it means. If I do, it's almost always during the reception of the Eucharist, but otherwise it's just the way we end our prayers.
It was surprising, then, to find that at the end of the explanation of the Creed in the Catechism there is a whole section on the word "amen." You'll find it in numbers 1061 through 1065. "In Hebrew," the Catechism tells us, "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."
I had always heard that "amen" means "so be it," that it is our "yes" to what has gone before, but this definition enlarges the content of our response. A bit further on we read, "Thus the Creed's final 'Amen' repeats and confirms its first words: 'I believe.' Our profession of the truths of our Faith is enclosed within our profession of belief.