The porn video tag "black-tranny-twerk" consists of three words:
1. Black: Refers to the ethnicity or skin color of the performer in the scene, specifically indicating a transgender woman of African descent.
2. Tranny: A slang term that stands for "transgender," referring to the gender identity of the performer, which is a person who was born as male but identifies and expresses herself as a female.
3. Twerk: A dance move popularized in modern times, characterized by rhythmically moving one's hips and buttocks in a provocative and sensual manner.
So, this tag represents a porn video featuring a black transgender woman (or more than one) who is twerking or performing a twerk dance for the viewer's pleasure.