A Painstopper is a brain implant that will completely eliminate a colonist's pain and any pain-related mood effects. Note that colonists with the Masochist trait get a mood bonus from pain, and this will also be removed by the Painstopper.
Most kinds of pain are temporary and not worth installing a Painstopper for. However, pain from scars is permanent, and some painful diseases such as Sensory Mechanites can linger for long periods of time.
Be careful when using this, as colonists with this implant will not go into shock from pain and will keep fighting until death (most common case), or be incapacitated from serious leg or brain injury.