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Squelch is a dial that changes a threshold below which analog radio signals are silenced so you can ignore noise. The dial allows you to dig into the noise when you want or be more conservative and only pass strong signals through.

Sets a minimum on the incoming signal to be amplified. For instance, only amplify stuff above x dB, silence stuff below x dB (noise).

A noise gate?

Not quite. More like signal-to-noise-ratio gate. In radio transmissions, there is white noise when no active signal is received. Radios mute themselves when there is white noise, as to not annoy the user. On 2 way radios this is very important otherwise the radio will be hissing at you most of the time.

The squelch setting determines the threshold of signal to noise allowed through. If the incoming transmission signal strength is really bad, the radio might not unmute itself. So you turn down the squelch, which might completely open the radio bringing in white noise, but you can then receive the transmission.

Isn't they exactly what a noise gate does? You set a level of loudness below which it mutes all sound. If sounds levels go above that then it plays whatever sounds goes above it.

Silence