What the heck is squelch and what does it do?

What should you set this to? Ive seen it on every scanner and i have no idea what it does(other than make noise if its turned too far to the right).

To make it easy, it loosens up your reception to hear farther away. It is normally set to just a hair into the quiet from where you hear the static. The static can change location depending on other interference too. Not only is the static annoying to listen to, but I've never seen a scanner that will scan with the squelch opened up loose enough to hear the static.

To a scanner receiver, even static is considered a live signal, and will stay there until the signal/static is quieted, or "squelched".