If you are playing an instrument that does not require you to use air, then you should phrase it as if you were singing it or playing on a wind instrument.
I've seen them used in piano music to denote a breath like pause in sound to contribute to the phrasing (usually to accomodate text though).
Yes, it means breath, so for woodwind, brass or singers they would take a breath, and the players would hold the silence until inndicated by the conducter to continue.
