A small rubber device inserted into the string bed of a racquet to reduce vibration and change the sound of the ball hitting the strings. Purely a feel and preference thing. No measurable performance benefit, but players swear by them. They fall out constantly during play and you'll see players pick them up or ask ball kids to retrieve them. Djokovic doesn't use one. Nadal does. This is treated as meaningful information by fans.
"The dampener flew out on that last shot. He signals the ball kid to grab it. Can't play without it."