Target Volume

The Target "Normal" Volume is the average volume you want each of your mp3 files (in Track mode) or each of your mp3 collections (in Album mode) to have.

The default is 89.0 dB because most mp3s will not have clipping at this volume level.
("Clipping" means that when the mp3 file is decoded by your player, some of the sound samples will be too loud. The player will "clip" these samples so that they do not exceed the maximum allowable value. This clipping creates a sort of rough, "scratchy" sound during loud parts of the song.)
If most of your mp3s are from very modern CDs, then you might be able to push the Target Volume up higher without introducing clipping into your files.