MLK stands for "Milk" in this context, which is a reference to the act of breastfeeding or lactation fetish. In a porn video tag, it indicates that the content involves either actual breastfeeding or roleplaying it as a sexual act. This may involve scenes where one partner nurses from the other's breasts or simulates nursing. It is associated with BDSM and kink communities due to the submission and control aspects involved in the act. Note that this interpretation may vary depending on specific context.