EQ (equalisation) is a fancy tone control. Instead of simple Bass/Midrange/Treble three-band controls,equalisers have 6, or 8, or as many frequency bands as you can pay for. Used before and/or after an effect (like distortion), this gives you more ways to contour the sound. Settings like the classic 'scooped mids' used for metal are easier to do with a good EQ.