An equalizer (EQ) lets you fine-tune specific frequencies of the sound spectrum, much like the treble and bass controls of a stereo, but with even more power and flexibility.
By adjusting the different frequencies, you can customize your music playback for different musical genres, listening environments, and output methods (such as speakers or headphones).
The horizontal values on the equalizer represent the spectrum of human hearing in hertz (hz). 32 Hz represents the deepest frequency (bass), 250 Hz and 500 Hz represent the mid-range frequencies, and 1 kHz to 4 kHz represent the higher frequencies (treble).
The vertical values on each bar are represented in decibels (dB), units that measure the volume or intensity of each hz frequency. By moving the sliders (also called "faders") up or down, you increase or decrease the sounds that you hear in that particular frequency range.
Preamp is a secondary volume adjustment that is applied to all frequencies equally. You might want to increase or decrease the preamp volume when listening to music that was recorded particularly quietly or loudly.
iTunes lets you adjust the equalizer settings yourself, or choose from over 20 presets of the most commonly used EQ settings.