Why do we say, "amen?" We read it in the Bible. We say it when we pray privately, and when we pray together. We find it many times in our worship service. It used to be common to end every hymn with a sung "amen." Some might even be inspired to shout it in response to part of the sermon! What does this mean?
Literally, the word "amen" means "truly" or "verily." When we say this in response to something someone else has said, it means, "I fully agree" or even, "may it be for me as he has said." It is as if someone has issued a statement and we are, so to speak, signing our name to the bottom to demonstrate our agreement.
Interestingly, Amen is more appropriate as a response by others than as simply a conclusion by the one speaking. If someone is praying, we might agree with them in prayer by saying, "amen" when they have concluded (or even during their prayer, in a subtle and quiet way). If someone proclaims a needed or particularly poignant word in preaching, someone might call out, "amen!" to show their embrace of that word. It is an act of participation. This is why those leading a group in prayer might conclude with an invitation to the group to say, "amen" by saying, "and all of God's people said..."