signal proccesing is divided into three categorie 1) frequency based 2) dynmics based 3) time based.
Equalizers boost or cut specific frequencies in a signl as they also allow to change the tonal balace of whatever you are controlling. They are divided into ,Graphic EQ-is controlled with sliders which boosts or cut each frequency band and Parametric EQ help in reducing feedbacks. The work of n engineer is make listener get the best sound