Up until this point in the series, it is unclear what exactly Jason is. Like, for all the viewer knows he may be a living, breathing human, albeit a freakishly ugly one that was thought to have drowned thirty years prior. But there's only so many times you can stab and electrocute and smash a living, breathing human before the writers have to come up with something else.
He's dead at the beginning of the movie, which takes place immediately after Friday the 13th Part 3-D, and then suddenly he's up and about killing a nurse and a doctor… so… that makes him what, an undead ghost zombie thingy? Whatever he is, this is the first time in the series he is so.
This is also the first time a Friday the 13th movie has some enjoyably gory slaughter, thanks entirely to the special effects wizardly of Tom Savini, who was under the impression this was to be the "final chapter". Poor Tom Savini.