Basically, it is the unintentional sounds that are made by the fingers repositioning themselves on the strings when the player changes chords or notes.

Artificial and/or “pinch” harmonics make that squeal. It’s actually included in some musical notation.

Roy Clark never played with the squeak. I think it would be a lot easier to play guitar with a squeak than without it.

It depends as well on the texture/composition of the strings. A smooth string (like Nylon) would make little sound while a wrapped string (like steel) would be more apt to make noise.