What is Auto Squelch? Pleaes Help.

Hi, I have just bought two Uniden UH-044 hand held UHF radios.

On one of them I am constantly getting hissing noises that can go on for quite a few seconds after a communication is received or given or even when it is idle. I understand this maybe people at a distance trying to communicate. However the other radio doesn't behave like that, it simply cuts out if a transmission can't be received properly.

Is this something to do with the auto squelch that is in these units? Is is possible that the auto squelch in the second one is not functioning correctly. I have been testing the radios a bit together and this seems to be the case. Both sets have freshly charged batteries too. Is this auto squelch something that can be adjusted or fixed if it is the problem?

I am new to this sort of things and slowly getting to understand how things all work. It would be great if someone could help me out with this and possibly solve my problem maybe.

BTW the one that is not functioning properly is a second hand one from the same shop where I bought the new one. They told me it was working fine. I just want to try to understand what might be the problem so when I go back to them I know what I am talking about.

Thank you again!