What does "Amen" mean? 

A word that at least a billion people have used but who knows what it means?

It's Hebrew and at the end of a prayer it means roughly "So be it" or "I agree"!  But that is not the end of it.  It has a broader meaning than that.  When Jesus said:  "Verily, verily, I say unto you ...." (e.g. in John 5:24), what word do you think he was using according to the original Greek text that was translated as "verily"?  That's right.  He was actually saying:  "Amen, amen, I say unto you".  So it's basically just a way of emphasizing the correctness of something.  

I must admit that I was rather staggered myself when I wondered what the obsolete English word "verily" stood for in the original text and found myself staring at "Amen" when I looked up my authoritative Westcott & Hort text.  I couldn't believe my eyes for a minute.  I even checked it in the Griesbach recension as well.

On further checking in my Abbott-Smith lexicon I see that the word was also used in the Septuagint:  The translation into Greek of the OLD Testament that Christ and the Apostles usually quoted from.  So we see how a Hebrew word got into Greek.  It has no exact translation into Greek so the learned Jewish translators of the OT in olden times simply reproduced it.  Abbott-Smith offers "be firm" as the meaning of the Hebrew original.

Even my Liddell & Scott lexicon of CLASSICAL Greek gives the word  a brief mention, maybe because of its Septuagint usage. We learn every day.