EQ is short for Equalisation, It is used to boost or reduce certain frequencies in a sound. Traditional analogue mixing desks have 3 bands a low, mid and high, this allows the mixing engineer to mix the low, mid and high bands independently to transform the sound into what the engineer want's.
There are numerous types of EQ available these include shelving eq, graphic eq, parametric eq, linear phase and filters such as low pass, high pass, band pass & cut, dynamic eq & more, all eq's do a slightly different job & it depends on what your needs are.
This is where you set the point of what frequency you would like to alter, cut or boost.
Cut / Boost/ Gain
Cut or Boost the frequency that you have chosen, you may need to adjust the gain afterwards due to either boosting or cutting the frequencies.
Parametric EQ's width (Q) changes the overall width of that which frequencies have been selected. The bigger the value of Q the narrower the area is.
This dictates how steep the slope drops away in dB, a 18dB slope means that 18dB worth of cut or gain will take place over a single octave.