Good heavens! You write music software and you don't own a music dictionary? It is as MMario says, with the addition that it can also be used similarly to extend a rest, and placed over the double bar it indicates the end of the piece. The meaning he gives is the one most commonly used in the notation of folk music.

Enclosure in !! also works with some programs, I believe, but not with the ones I've tried. Too many clashes with other usages, I suspect.