An equalizer (EQ) is used to boost or cut the volume of certain frequencies. Most equalizers have multiple bands, which enables the user to boost or cut volume across multiple frequencies, or bands. An equalizer that contains high, mid and low frequency controls is referred to as a three band EQ. Equalizers are used to shape sound of recorded content, correct poorly recorded audio, create a new sound out of an existing sound, and to blend multiple instruments into a well-balanced audio mix. It is highly important to monitor your audio on high quality flat (as in no volume boosts or cuts between 20 Hz to 20 kHz) speakers or headphones while working with EQ. I tend to use EQ sparingly unless I am attempting to salvage poorly recorded audio.
There are several types of equalizers the most common EQs you will see in most video editing software are: Graphic EQs, Parametric EQs and Shelving EQs. These three types of EQ are important to know, because they are common to video/audio production software, as well as recording studios or post houses.
Graphic Equalizers provide multiple volume boost/cut controls for a wide range of pre-selected frequencies. The graphic EQ is easily distinguished by its controls, which are typically a series of sliders that are physically lined up next to each other, providing a graphic representation of the overall EQ curve.