The diagram to the left represents the majority of mixing consoles found in use today.
This channel strip features a Low Frequency Shelf, a Low Mid Frequency Shelf set at 250 Hz, a High Mid sweepable and a High Shelf.
Consoles differ on the set points but all shelving eq's do the same thing. They boost or cut the gain of a certain frequency. The boost increases the amount of gain below (Low) or above (high) the factory set frequency point.
A sweepable eq gives one the ability to set the frequency to be effected by the boost or cut. On a typical console the sweepable eq will increase in width as you boost the set frequency. Same is true for a cut of the set frequency.
The most useful frequency attenuation on a console is the High Pass Filter or HPF. This friendly button and attenuator allow the user to cut or "shelf" frequencies out of the signal path prior to the eq section. Why is this useful? Female vocals, acoustic guitar, flute.. all produce frequencies above 150 Hz and more importantly do not generate these low frequencies (we call it mud). By removing these lower frequencies from sources that do not generate them you are allowing the low frequency instruments to occupy that space alone. Low frequencies take the most energy to generate.
Some consoles do not have an attenuator for the high pass frequency. In this case the set HPF frequency is 75-80 Hz
This channel strip also has a Low Pass Filter.