A guide finger is simply a finger that keeps in contact with the string while shifting. This is going to give you some extra security and accuracy in the shift, and it can even mean that you don't have to look at your left hand when moving about the fingerboard.

“Piano hands” are so used to playing piano that they naturally take up the correct hand position at a keyboard. Time spent practicing has made the fingers strong and quick. A player with piano hands can control the speed of their fingers to get beautiful variations in volume and articulation as they play.

Finger positions are the key to learning to play the piano quickly and well.