A notch filter has a very high Q (narrow bandwidth), attenuating a single frequency by 6–30+ dB while leaving adjacent frequencies untouched. Uses: removing electrical hum (50/60 Hz and harmonics), taming a specific room resonance, and eliminating feedback frequencies in live sound.
In live sound, 'ringing out' monitors involves finding and notching the specific frequencies that feed back. A graphic EQ with narrow filters is used to cut these frequencies by just enough to prevent feedback without audibly affecting the overall tone.