A notch filter is a filter that filters out a particular frequency from a signal. So say i have a a bunch of signals with many frequencies from 1-100hz and send it through a notch-filter designed to filter 50hz i would notice that the signals around the 50hz get a lot weaker. So in the Frequency spectrum a "Notch" appears in the middle! Hence Notch-filters!You may think "what is the use of that?" so here is a Application example:Say you have a audio-amplifier with a simple power-supply connected to your main. Because the main tends to alternate at 50-60hz it is possible that tiny distortion from your supply gets in your audio amplifier (causing sound to Hum). By using a Notch filter designed to filter out 60hz you can remove the unwanted Hum!