Hi/Medium/Low Equalization   

  Most mixing board input strips contain High, Low and Mid-range frequency control pots (some more sophisticated mixers may have elaborate EQ including "hi-mid" and "low-mid" settings). 

While the EQ on a mixer is usually too crude to do elaborate equalizations, it is useful for amplifying or attenuating the general areas of the spectrum of the input sound. If, for example, one wished to remove some hiss from a sound, attenuating the High frequencies control pot would help.