Each band in a quad equalizer is a filter tuned to a specific frequency, and it can turn that frequency range up or down independently of the rest.
Two filter shapes cover the four bands: shelving filters (used for the low and high bands) adjust everything above or below a point, like tilting a shelf. Peaking filters (used for the two middle bands) target a narrower range centered on one frequency. Stack all four, and you get independent control across the whole audible spectrum.
When you load a file, your browser decodes it and routes the resulting audio signal through four BiquadFilterNodes in sequence — one per band — using the Web Audio API. Each node applies its own boost or cut before passing the signal to the next, and finally to your speakers or headphones.