An equalizer or EQ allow you to boost or cut certain frequencies. Some EQ's only have a very few bands, other have more than 10 bands giving you more detailed control.
The most basic EQ is the Graphic Equalizer, where you have a fixed set of sliders each controlling a fixed frequency. You have most like seen one of those on some HiFi equipment.
The Parametric Equalizer give you more control by allowing the user to modify Level (boost or cut), the center of the primary frequency and the bandwidth or range of frequencies.