Hidden Track One Audio
Hidden Track One Audio (HTOA) is audio content that is labeled Track 01 Index 00. To play this content, it is usually necessary to scan backwards from the "start" (Index 01) of the first track on the disc.
When ripping CD audio to individual tracks, it is common practice to append the audio content from each track's Index 00 -- also known as the "pregap" -- to the end of the previous track. As there is no "previous track" to append Track 01 Index 00 onto, it may be necessary to use special steps to extract HTOA content from a CD.
See Also
Table of Contents Wikipedia: List of albums with tracks hidden in the pregap