We all have a very good idea of what volume is. For the sake of definition, in music, it is usually regarded as the degree of loudness or power behind the sound. Contrary to the overall volume in an audio file, the Track Volume is mostly found in video files as well as Audibles AAX file format.
These files usually contain several audio tracks. In stereo files, these might be a left and a right track. For videos, this can also refer to different audio tracks in different languages. These tracks, naturally, can have varying volume. While the overall volume can be changed according to preference using media players, the Track Volume is set inside the file by the program that was used to create the file.
Typically, the volume is either set to 0% or 100%, but variations may occur.
When checking your video file for hidden metadata or exif information, the Track Volume is usually paired up to the track it describes. At the very least, it is referencing the correct track in some way.