"Stiff" in the context of a porn video tag typically refers to an erect penis. It is often used to describe the physical state or readiness of the male participant(s) for sexual activity. In this case, it is a common term used by English speakers and indicates that the man in the scene has achieved an erection, suggesting he is prepared and engaged in sexual activity. This term is commonly used in the porn industry to describe the physical state of male performers.