"Bottoming out" is a term used to describe when a key stroke is pressed all the way down where the stem of the switch hits the inside of the switch casing. This causes a "clack" from the collision on mechanical keyboards. Switch sound dampeners are soft rubber O-rings that can be installed onto every single keycap stem to cushion the landing of the switch by stopping the key stroke just short of the point of bottoming out. This also very slightly reduces the max travel distance of the switch.