you know, like on a visual equalizer, how when the music is playing all the different lines represent different frequencies.
the ones farthest to the left are for low frequencies, the further you go to the right the higher the frequencies are.
if you had ALOT of bass and not good highs or midbass then your lower frequencies (the ones furthest to the left) will be peaking alot higher than the higher frequencies simply because you have too much bass.
a flat response, with little peaks, will be nice and even all the way across because it is all playing in harmony, instead of weak tweeters and door speakers and 4 15s in the back.
visual equalizers are on almost any car reciever, home receiver and even media players on your computer...