It's a transport like the hiface or audiophileo 1 2 - the bridge between your computer and the dac. I couldn't justify using it with the nfb12 ($200 dac $200 transport
); but the Nfb3 is worthy enough of a decent transport imo.
You can read various thread here to see people mention many "improvements" transport can bring: bigger soundstage, more resolving, enhancement on stuff and stuff. I don't have much experience with said transports; however upon trying various input on my Nfb12; I'm convinced that I need one (the optical out on my mainboard murders the soundstage; usb sounds a bit "flat", coaxial sound best, adding a bit of warmth to the sound, they're small differences but quite noticeable). I'm excited to try what improvements a dedicated transport can bring.