equalizing music sound levels on all ipod songs

Hello,
I noticed there was sort of a "adjust sound levels on all songs" option in itunes.
If I decide to do this, does it automatically and permanently change the sound levels in every mp3 track on my ipod? If so, would it make all of the lounder versions quieter, or vice versa?
It sometimes is annoying when one song is so quiet and the next is blaring; but I do not want to change all of my songs to the level of the quiet one.
Can you help explain why this is and what to do about it?
Thanks,
M