My wimpy overview of audio equalization (EQ):

An equalizer (EQ) lets you independently adjust the relative volume of different frequencies in your sound. While most car and home stereos let you adjust low (bass) and high (treble) frequencies, an equalizer gives you more fine-tuning. For example, road noise in a car is often in the 25-200 Hz  frequency range (HZ is cycles per second), so an EQ would let you compensate by reducing the frequencies in that range. Without an EQ, you'd have to turn the treble down, and strip out the good with the bad. Someone doing a podcast, for example, might discover a background hiss and use an equalizer to remove ONLY the frequencies causing the unwanted noise, keeping the rest of the high frequencies intact. An audio person might hear the noise and say, "I can EQ that out."

The key point to understand is the nature of the scale -- zero is in the middle. If you have audio that is not "EQ'd", all the sliders are at the zero/middle point. While the frequency bands themselves are absolute (20 hz is 20 hz regardless of the volume), the vertical scale is relative to the current "master" volume level. So if I make big adjustments to the sliders, those adjustments are always relative to the current volume level.