This porn video tag refers to a specific sexual act involving a transgender individual. "Tranny" is an abbreviation for the term "transgender," and in this context, it is referring to one of the participants in the scene being a trans woman. The phrase "cum-mouth" indicates that there is a focus on the act of another participant ejaculating onto the trans woman's mouth. So, overall, this tag signifies a video where a transgender individual performs oral on someone who then ejaculates into their mouth. This is explicitly intended for adult audiences and viewers familiar with explicit pornographic content.