French for good (bon) word (mot). As such, it should generally be pronounced as bon moz. (with the 'n' is very soft or absent). I can't speak for elsewhere, but here in America we usually just say all the letters, and them foreigners be damned.
'Bon mot' refers to a clever or witty comment or saying. Those of smaller vocabularies would call these 'quips' or 'witticisms'.
The plural is Bons mots.