Hi Rocky,
NAS is a function built in to the B2. NAS is also provided in PCs, Macs, Chromebooks etc. With NAS enabled, the B2's internal hard disk drive 'Music' folder will show up on your PC's Windows Explorer (or Mac Finder) as just another folder. So, from your PC/ Mac you can drag and drop music folders and files that are really on the B2. You can edit your artist, album and track names without touching the B2.
It is a hugely useful feature if, like me, your music collection needs to be cleaned up. It is so much quicker making the changes from a PC/Mac than trying to use the WebUI or the B2 front panel.
If you look at the attached jpg file hopefully this will help. This is a screenshot of Windows Explorer on my laptop. Marker 1 shows my B2 showing as 'Brennan-B2' (I have two B2s so the 'BrennanB2-1' shows just above this entry. Marker 2 points to a list of the B2's artist folders. So the NAS functionlets you cess the B2's 'music' folders directly, as if those folders were on the PC.
Regards,
Peter.