"Nut" in the context of a porn video tag generally refers to ejaculation or orgasm. It is commonly used to describe a scene where characters reach climax, either through sexual intercourse or solo masturbation. The term is derived from slang for ejaculate (cum) and has become a popular way to label videos featuring climactic moments in adult content. It's also frequently seen with other tags, such as "nut on boobs" or "nut all over."