This porn video tag is describing a specific type of adult content. Let's break it down:
1. "big": Refers to the preference for a larger body part, in this case, referring to the buttocks or hips of the performer(s).
2. "booty": Another slang term for the buttocks. This word emphasizes the focus on the lower half of the body and derriere.
3. "tranny": Short for "transgender," this term refers to a person whose gender identity does not match their assigned sex at birth, typically referring to someone who transitions from male to female (MTF).
So, this tag is describing a porn video featuring a transgender performer with a large buttocks or hips. The content caters to viewers who enjoy adult films with transgender performers and have a preference for larger buttocks.