It's like the 3rd game in the main story universe so it kind of assumes you know all the characters and world already
Neptunia is sort of weird in how its set up, basically there's a bunch of canon parallel universes (and people can hop between them). Re;Birth 1 is its own story and introduces all the characters because its basically intended to be the first game. Re;birth 2, Re;birth 3 (A remake of Neptunia Victory), and VII (V-2, as in Victory-2) are the current 'main plot' and all take place in the same universe.
Not really recommended to start with Re;Birth 2 though (even though you could) because its the weakest game in the series both character and gameplay wise.
Of course if none of that bothers you you totally can just jump in wherever, though.
Thanks for all this; Ill pick up Re;Birth 1 on Vita when i get a chance ---