Question: What do a parrot and a walk through a bog
have in common?
Answer: They both make noises which are common radio terms in aviation.
In plane English (see what I did there!):
Squawk is a four-digit code given to the pilot by air traffic control (ATC). This establishes the aircraft on the air traffic controller’s radar screen and turns the random blip to an identified aircraft.
Once the pilot has been given the squawk code, he enters it into the plane’s transponder (a short of radio transmitter) which then emits the code to the world. Transponders have different ‘modes’ and can provide other useful information such as location and altitude.
Squelch is a function designed to minimise unnecessary radio noise. If you’ve ever heard the hiss and crackle of a transistor radio which isn’t quite tuned into the station you’re trying to listen to, this is what you’d get without squelch.
Ideally, squelch should be turned up just enough so that the background noise disappears and the station comes through nice and clear; too much squelch and you risk tuning-out the station you’re trying to listen to, too little squelch and you’ll have that constant hissing and crackling in the background. If the signal is weak, squelch will need to be minimised in an effort to get as much of the signal as possible.
A Pilot's Companion 