"Seeding" in the context of a porn video means that the actor is ejaculating. In this case, it refers to the act of ejaculation occurring while the actor is penetrating another performer "inside". So, "seeding-inside" describes a scene where an actor is ejaculating while they are inside another performer. This typically implies that the actor is having vaginal, anal, or oral sex with the other participant and reaching the climax by seeding (ejaculating) inside them.