Determining Song volume

It means that you have Sound Check enabled in iTunes > Preferences > Playback, and when songs are added to your iTunes library, the software automatically scans the files and edits their tags to include information about volume adjustments so that songs play back at relatively the same volume. It can be thought of, in other terms, as "track gain," as opposed to "album gain." This means that albums will no longer play back with the original differences in perceived loudness, so an album with some quiet tracks and some loud tracks will lose these intentional nuances.

If you like to listen to albums, I would recommend disabling Sound Check. If you like to shuffle your songs or create mixed playlists of songs from different albums, you may find that you like the effect.

Disabling Sound Check will keep iTunes from using the information stored in the songs' tags on playback. When Sound Check is enabled, it scans all the music in your library and writes new and additional information into the tags of each track. There is no way to use it for a partial playlist: it's an all or nothing deal.

You really might want to check out MacMP3Gain as an alternative to Sound Check. It works better, as it uses ReplayGain instead of normalization like Sound Check, and is versatile in that you can use it for Track Gain (probably what you would want in your case) or Album Gain (what I use since I like to listen to albums as albums--no shuffling, etc.).