"Trans" stands for Transgender, and "BBM" typically refers to Big Black Men. In the context of a porn video, "trans-bbm" means a porn video featuring a scene with a transgender performer (someone assigned male at birth but identifies and presents as a woman, who may have physically modified their body in various ways) engaging in sexual activities with one or more black men. This tag is used to help viewers find content that specifically includes both transgender and black men participants.