Parametric Equalizer

Basically a bunch of potentiometers [variable resistors] otherwise know as volume controls, each based around a frequency band (with the use of capacitors), that allow you to boost or repress sound signal levels at various frequencies throughout the audible range.

Let's say you have a room (or recording, or SL2000 tweeter in your system) that seems too "bright" at 12,000Hz. Find the control for that frequency range/parameter (therefore, parametric) and turn it down a bit.

Maybe your room soaks up low bass. Find the the control for that range of frequencies and turn it up a bit, etc., etc.

A balanced system/room should produce all frequencies at an equal level given equal input power (flat response), but that's hard to achieve in the real world. An equalizer is an attempt to boost or soften the frequencies not being reproduced equally by your speakers and electronically create that flat response curve.

The problem is all of those components/controls in the equalizer add TONS of noise to your music unless you spend what a top recording studio would spend for an equalizer.