Index marks are a feature in the CDDA (audio CD) standard intended for use by classical albums to mark sub-track breaks, e.g. arias in an opera.
It never really caught on, as few players did anything with them, and even fewer discs had them.
There can be up to 99 index marks per track on an audio CD.
They are apparently sometimes used as part of a CopyProtectionScheme; by using nonstandard gaps between audio tracks and leaving index marks in unexpected places to confuse or break certain types of copying software.