Why Squelch is a receiver circuit that keeps the audio output muted until an incoming signal meets a set threshold, usually judged by signal strength or by the absence of noise in the channel. Without it, an idle FM receiver pours out loud white noise (hiss) because the detector amplifies whatever noise is present. Turning the squelch control up raises the threshold so only stronger signals open the audio; turning it down opens the receiver to weaker signals along with more noise.
Watch out Reducing impulse noise is the job of a noise blanker, which works on received signals while they are being heard; squelch simply decides whether you hear anything at all.
Squelch = silence until someone talks. It gates the speaker, it does not clean up the signal.