"Tranny" is a slang term for a transgender person. In the context of adult content, it can refer to scenes involving transgender performers or scenes where performers are transitioning or have gender dysphoria.
"Jacking off" is another term for masturbation, which is the act of self-stimulation to achieve sexual pleasure or orgasm.
So, "tranny jacking off" would refer to a porn video featuring a transgender performer (or performers) engaging in masturbation for the purpose of adult entertainment.