Disc ID is the code number which MusicBrainz uses to link a physical CD to a release listing.
It is a string of letters, like XzPS7vW.HPHsYemQh0HBUGr8vuU-.
Disc IDs for a release can be seen on the disc IDs tab. Clicking on these will give a detailed display of the disc ID.
Disc ID Clashes
A release may have any number of disc IDs, and a disc ID may be linked to multiple releases. This is because disc ID calculation involves a hash of the frame offsets of the CD tracks.
Different pressing of a CD often have slightly different frame offsets, and hence different disc IDs.
Conversely, two different CDs may happen to have exactly the same set of frame offsets and hence the same disc ID, for example lwHl8fGzJyLXQR33ug60E8jhf4k-.