The fine art of dampening strings, or specifically stopping particular notes from ringing and thereby colliding with the other notes that you do want. I learned much about this by watching Jon play bass. The fingers of both of his hands are constantly refining the sound that comes forth from his instrument, adding a slow vibrato here and dampening a string that would otherwise clash with the next harmony.
You can observe this constant vigilance in classical guitarists like Julian Bream. While one finger of the left hand goes to a fret to define the next note, another finger is poised to dampen the string that rang the last note.

I recommend renting a DVD of Bream playing guitar as it is most interesting and educational. (((You might also observe how he bends certain notes to create harmonies that are in tune… the well-tempered scale is a compromise, especially on a guitar, and you will notice when you play an E major chord followed by a C major chord that the G-string, if tuned for the E chord, will sound off when playing the C chord and vice versa.)))

And the faces he makes while playing guitar are very entertaining, also.

This, of course, is most important when changing keys, but is always a good idea because even strings you haven’t plucked or struck with the right hand will ring sympathetically. By dampening those strings you focus more attention to the notes you are playing. Things become clearer, as if a fog has been lifted.