It means that sound and picture are not recorded to the same media - this is common in film where you may use a DAT recorder or anything else (mini-disc, or whatever). It is perfectly acceptable, and it just means that you must keep copious track of shot/scene/tape(reel)/etc by way of slate - - - - - this is the original use of clapping the slate so that you can match the action of the slate hitting and the sound spike in your sound files what you've recorded.