An equalizer is a sound modifying module that helps to balance the frequencies of the incoming signal. The equalizer changes the spectral content of the sound. We build equalizers from a combination of filters and gain. We often use equalizers to make multiple sound sources combine together clearly by avoiding overlapping frequency ranges.
The typical ranges are: low bass, mid bass, upper bass, low mids, high mids, and highs. Each of these ranges has unique properties for human hearing. For example, too many low mids can make the sound dark and muddy. Too many highs will make the sound harsh.
An equalizer is essentially a collection of different types of filters. Each filter has the same properties as a normal filter: type, cutoff (or frequency), q (or slope, range), and gain.