Amen means this is true and comes from faith that dares to pray – The Largely Catechized Life #68

“But all depends upon this, that we learn also to say Amen, that is, that we do not doubt that our prayer is surely heard, and [what we pray] shall be done. For this is nothing else than the word of undoubting faith, which does not pray at a venture, but knows that God does not lie to him, since He has promised to grant it. 120] Therefore, where there is no such faith, there cannot be true prayer either.“