The 851N's primary function is to play music over the network, that you have either ripped from CD or downloaded. Spotify and Airplay are two alternative convenient methods of playback.
You can copy your CDs by using any ripping software - you can even do this in iTunes.
We would recommend ripping files in formats such as FLAC. This will ensure that the files support things like album art and album/track info (metadata).
Once these files have been saved either on your computer or a NAS drive, they can be accessed within your network via the Cambridge Connect app.