(1): Good pain and bad pain are terms used lightheartedly by BDSM practitioners, signifying that whilst BDSM may include an element (often quite pronounced) of consensual pain, there is a purpose to it, and some pain is consented to and accepted whilst other pain is not. “Good pain” is therefore pain that is mutually agreed, desired or permitted by the submissive partner to be experienced, and seen by them as of enjoyment or value.
(2): Good pain and bad pain refer to perception of pain as pleasant vs. unpleasant. Sensations that non-practitioners imagine to be painful are instead perceived and described by BDSM practitioners as pleasurable or a good form of pain, in much the way that muscles after a workout at the gym may be sore, but in a good way. The transition of perception from “bad pain” to “good pain” may require a warm up beforehand.