The EQ (equalization) section is used to adjust the amount of boost or cut (attenuation) for high, midrange and low frequency bands (the exact number of frequency bands varies according to the mixing console model). Certain mixers allow the user to adjust the exact frequency of the bands as well as their boost or cut. For example, on this mixer, notice that the LO MID(range) section allows adjustment of the center frequency (lower pot) as well as the amount of boost or cut (upper pot). This is called a parametric EQ. A common novice mistake is to twirl the lower knob while leaving the upper knob set to zero, which will do absolutely nothing to the sound, since you are neither boosting nor cutting the frequency band.
The high mid(range) section of this mixer adds a third control (lowest knob) to the boost/cut and center frequency, namely the bandwidth (or Q), or frequency width of this band in octaves. Neither the lowest nor highest band is adjustable, just the amount of boost or cut. Both the high mid and low mid are peak/notch filters.
The Hi and Lo EQ use shelving filters, meaning that they boost or cut most frequencies above 12K (Hi) or below 80 Hz (Lo) equally (picture a horizontal shelf moving up or down without tilting) with a small transition slope.
In addition to the band EQs, mixers may provide a high-pass or rumble button to eliminate unwanted low frequencies caused by vibrations on a mic stand, foot tapping, HVAC, etc. In this case, depressing the low cut button will roll off frequencies beginning at 75 Hz. Finally, be sure to depress the EQ IN button if you wish your EQ settings to have any effect. Toggling this button is a good way to ascertain the effect of your EQ on the sound vs. the original signal.
Equalize tracks separately before mixing down a multitrack recording. One caution, however, is that boosting or cutting frequencies will cause the input balances to change. Extreme boosting of frequencies, particularly in the low band, may cause a channel to distort, since it may add up to 15 dB of energy, more than four times the original strength.
Finally, your ears are your best guide, but there are many, many tips on EQ out there on the web. While some are geared more towards a pop sound, even then you may get some good pointers. And be creative with EQ—mentioned earlier were ideas about having different EQ settings on your effects returns.