Wikipedia says the word is a declaration of affirmation meaning “So be it; truly.” It is found in the Hebrew Bible and the New Testament, and is a concluding word for prayers and hymns. Simplybible.com says when it is used at “…the end of a prayer it expresses the genuineness or truth of the petition or praise. Amen means ‘truly’ or ‘fair dinkum’.” It also is used colloquially to express strong agreement, as in “amen to that.”