This porn video tag is describing a scene involving two transgender women engaging in lesbian sex. The term "trans-girl" refers to the transgender identity of the women involved, while "on-girl" indicates that the interaction is between two individuals of the same gender. In this context, it implies a sexual encounter between two transgender women, with both parties being the active participants in the act. This tag is specifically targeted towards an adult audience who are interested in and experienced with porn involving transgender women.