Amen is a Hebrew word meaning “so be it” and it is generally expressed when we strongly agree on what has been said.
You may sometimes hear it being uttered in jest but this shouldn’t be for it is also a name given to Jesus as written in Rev 3:14 at his Message to Laodicea:
"And to the angel of the church in Laodicea write: The words of the Amen, the faithful and true witness, the origin of God's creation:"
Lastly, Amen is generally used in the mass as a concluding word for prayers and hymns. This is not surprising because Amen is the last word written in the Bible:
“The grace of the Lord Jesus be with all the saints. Amen” (Rev 22:21).