Frequency and Level
One of the most important features of sound is frequency. Imagine one string of a guitar. When plucked, it vibrates at a certain rate. This is its frequency. The lowest string of a guitar vibrates approximately 82 times per second, or 82 hertz (Hz) to be technical.
The human ear can only hear a certain range of frequencies. 20 Hz, or twenty vibrations per second, is about as low as it can go. The human body can perceive frequencies lower than that, but it is not hearing them in the true sense, but rather feeling them in the abdomen. The upper range of frequencies that we can hear extends to 20,000 Hz, or 20 kilohertz (20 kHz) – 'kilo' is the abbreviation for one thousand, just as one kilogram equals one thousand grams.
It is useful to put this into perspective in relation to musical instruments. The piano is a good reference point. The lowest note on a standard piano is 27.5 Hz, and the highest note is 4186 Hz. No commonly found instrument goes lower than the piano; the piccolo extends a little higher.
An equalizer will have controls to cut or boost high frequencies, and to cut or boost low frequencies. We call these the HF and LF controls, or simply 'high' and 'low'. You can control how much cut or boost to apply, which is galled the 'gain' control, and the frequency from which the change will start to be heard.
There will be a mid-frequency control too (MF or 'mid'). This will have a gain control for the amount of cut or boost, a frequency control for the center frequency around which the change will be heard. It has another control, called simply 'Q', which governs the width of the band of frequencies that are affected. Sometimes this control is called 'bandwidth'.
An equalizer that features all of the above is often called a 'parametric equalizer' and this is what you would find as standard in digital audio workstation softwares. There is another style of equalizer called the 'graphic equalizer' which has a separate control for each of 30 or 31 bands of frequency. Graphic equalizers are commonly used in live sound as they are very quick to operate and give an approximate visual representation of the changes made.