It is a string of letters, like XzPS7vW.HPHsYemQh0HBUGr8vuU-, and on the release summary view are disc IDs denoted with this icon .
On the release view page, any disc IDs are shown at the bottom of the page, and clicking on these or on the "View Info" link 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-.