This is a small amount of concave bow in a guitar or bass neck, usually created by an adjustment to the truss rod, resulting in a minimal amount of dip in the middle frets of the fingerboard, Relief is intended to allow room for the strings to vibrate as they’re played (strings vibrate in an elliptical pattern, with the widest excursion in the middle), without buzzing on the frets. Too much relief will result in high, stiff action and possibly compromised intonation, while too little may result in excess buzzing of the strings on the frets.