From Hydrogenaudio Knowledgebase
Hidden Track One Audio (HTOA) is non-silent audio data located in the index 00 portion ("pre-gap") of track 01 on an audio CD. CDs normally start playing a track at index 01, which comes after 00, so to hear HTOA, one has to manually scan backward from the start of track 01. Not all drives can access audio data in the pregap. Not all DAE (CD ripping) software supports reading such data, even if the drive does.
Not all CDs have an index 00 portion of track 01, but among those that do, it's usually just a small amount of silence. This can be called HTOA, but generally, when talking about CD content, HTOA really only refers to non-silent audio data. For example, "I have a CD with HTOA" implies something other than silence precedes the main part of track 01.
HTOA is also used to indicate whether a drive is capable of reading track 01 index 00 at all. For example, "My CD-ROM drive has HTOA support" means that it can extract audio from that region of the CD, regardless of whether it's silence.
When people refer to a "hidden track" on a CD, they may mean HTOA, or they may mean the more common situation where the last track simply contains two songs separated by silence.
 HTOA support in CD drives
Some computer CD drives aren't capable of reading audio data in the area of the disc prior to track 01 index 01 — some refuse to try, and some return false data (silence, even when it's not silent). This capability is independent of a drive's capability to overread into the lead-in or lead-out areas of the disc.
 HTOA support in DAE software
- Exact Audio Copy - If the CD's table of contents (TOC) indicates track 01 index 00 is greater than 6 seconds, the info for track 1 will be colored red to indicate that HTOA might be present. In order to extract track 01 index 00 (regardless of whether it contains HTOA or is colored red), you have to perform either 1. a full image copy, 2. an index-based rip, 3. a range-based rip, or 4. regular track copy but with gaps appended [sic; actually prepended] to next track rather than previous.
- DAE Drive Features Database - HTOA info and how to test a drive for HTOA extraction support
- DAE Drive Features Database - homepage where you can search for a drive's HTOA support
- Hydrogenaudio forum thread on this topic