From the help file:
Sympathetic Resonance - Sympathetic resonance is a subtle phenomenon that occurs in real pianos when the sustain pedal is pressed. Normally, when a piano key is not being played, a felt damper presses against that key's strings, keeping them from vibrating. However, when the sustain pedal is pressed down, those felt dampers are removed, and all of the strings of the piano are left to freely vibrate. When keys are pressed and notes are played, other nearby strings, because they are no longer dampened, start to vibrate in sympathy with the strings that have been played. This causes a slightly fuller piano sound to emerge, as more strings are vibrating (or resonating) around the ones that were actually played. When the sustain pedal is released, this Sympathetic Resonance sound is immediately muted.
Sympathetic Resonance Slider - This slider controls the amount of sympathetic resonance that is heard. This slider has a range of 0 (no Sympathetic Resonance) to 100 (very loud Sympathetic Resonance).