disc.id is a file found on some DVDs (or other disc types) identifying the contents. It is an ASCII text file consisting of name and value pairs one per line with equal signs, and a square-bracketed header. It is apparently used by the InterActual media player software.
One found "in the wild" has this content:
[InterActual] id=10000017000059000011 ver=1.00.02.0705 name=A Hard Day's Night owner=Miramax index=http://iti_cd_path\common\default.htm
The "id" field is apparently a unique numeric identifier, and "ver" a version number. "name" has the title and "owner" the studio or other content owner. "index" seems to be in the form of a URL, but in this case it is a badly-formed one, using the "http" protocol but not having a proper domain address, and then using backslashes in the path where forward slashes are supposed to be used.