The porn video tag "tranny-shower" is a combination of two words: "tranny" and "shower." In this context, "tranny" refers to a transgender individual, specifically someone who is transitioning or has transitioned from male to female. The term "transsexual" can also be used as an alternative.
"Shower," in the context of pornography, often means a scene where a person (or persons) is either taking a shower or engaging in some form of water play, such as water sports or wet and messy fetish scenes. This tag can be understood as a video depicting a transgender individual taking a shower, or engaging in water-based sexual activities.
Note that this tag description is for adult audiences who are experienced with porn videos and understand the meaning of specific tags.