The porn video tag "jizzonmygf" is a combination of three words: "jizz," "on," and "my gf" which stands for "girlfriend." This tag refers to a particular type of sexual act that takes place in adult videos, namely male ejaculation or "jizz" onto someone's girlfriend.
In this context, it describes a situation where a man is having sex with his partner and chooses to climax (ejaculate) on her girlfriend instead of inside her or elsewhere. This can be considered as a form of intimacy between the two partners involved, showcasing trust and an open relationship dynamic.