Amen means this is true and comes from faith that dares to pray.

“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.“