Why A notch filter is a very narrow band-reject filter placed in the receiver's IF or audio chain. It puts a deep null on one specific frequency, so a steady carrier or heterodyne tone sitting inside your receive passband is attenuated while the rest of the signal, including most of the desired voice, passes through. Many modern rigs include an automatic notch that hunts for and tracks such tones on its own.
Watch out Impulse noise like ignition pulses is handled by the noise blanker, not the notch, and splatter from adjacent signals is reduced with IF filtering, passband tuning or shift, since splatter is broad rather than a single tone.
Notch = one narrow hole for one tone. Blanker = pulses, IF filter = splatter.