Anything (a digital souncard, a digital converter, a usb device) that is bit-accurate means that the device transfers the digital data perfectly with no alteration whatsoever to the 1's and 0's. Another phrase that means the samething is Bit for Bit.
For example, I know the Soundblaster soundcards have digital inputs, but it resamples the digital info (meaning there is a change to the 1's and 0's) therefore the Soundblaster soundcard is not bit accurate.
In general you want to avoid digital devices that are not bit accurate. Some people will knowingly use them saying only the data is affected and not the sound, but I feel the technology as well as the prices are getting much better and therefore you can avoid such devices.
ps, there are ways of testing the bit accuracy of a device. I am not too familiar with such concepts but I know others are.