In the context of porn video tags, "skin" refers to an explicit scene involving human skin in intimate contact or being caressed. This typically implies scenes with nudity and close-up shots of bodies touching and rubbing against each other. It may also be used as a descriptor for scenes that focus on sensuality and physical intimacy without necessarily implying sexual intercourse.
For example, a video tagged as "skin" might include scenes of intimate caressing, stroking, or skin-to-skin contact between the participants in the video, often showcasing their natural beauty and focusing on sensual pleasure.