RSVP means that they want you to send an email or call them confirming that you're attending the event so that your name will be included on the guest list and they will be expecting your arrival.
You can come at anytime during the time stated in the invitation. You can stay for as little as 10 minutes or during the entire duration of the event, most store parties are very casual. No need to dress up, but if you want you can. It's basically a private shopping experience where drinks and small bites will be offered and you can have one on one with your SA or just mingle around without the interference of other patrons during normal business hours. It's basically a way for them to say "You are a (potential) frequent customer, here's an exclusive party invite that will make you will feel special and maybe this will guilt you into buying more from us"
Sometimes bigger stores will have parties hosted by magazines, fashion insiders or a preview or launch of new collection. That can be a bit more formal and the invite can get even more exclusive, usually based on how much you spent.