For information on this function you may want to check the example program shipped with LabVIEW and IMAQ Vision. You can find it in this directory:
C:\Program Files\National Instruments\LabVIEW 6.1\examples\Vision\3. Applications\Barcode Example.vi
Also you may want to drop the Barcode Reader VI into the block diagram and right-click over it, select Help. If this does not clarifies the question you might have please post the details of it.
Good luck!
Nestor.
Nestor