This tag is composed of three elements: "thirdworld," "xxx," and "trans."
"Thirdworld": This term typically refers to countries or regions that are economically, socially, or politically less developed than others. In the context of porn, it might suggest content featuring performers from such areas, or scenes set in those environments.
"xxx": This is short for "adult," and is often used as a label to denote explicit adult content. It's usually seen in the context of movies, images, websites, etc. intended strictly for an adult audience.
"trans": This refers to transsexual or transgender performers or themes, indicating that the porn video includes individuals who identify as such.
So, the overall tag seems to be referring to explicit adult content featuring transsexual or transgender performers from third world countries or regions