"FTM" stands for "Female to Male," referring to a transgender individual who was assigned female at birth but identifies as male. "Pussy" refers to the genitals of a female person. In this context, it implies that a cisgender man (not transgender) is engaging in sexual activity with a transgender woman's genitals. "Fuck" is a common term used for penetrative sex.
So, the tag "ftm-pussy-fuck" means a porn video where a cisgender male engages in penetrative sex with a transgender woman's female genitals. This genre of porn typically caters to individuals who have an interest or fetish for trans women and their unique bodies.