what does this actually mean?

Waitlist means you are not on the top of the list of candidates to whom they would like to extend offers, nor are sub-par enough for an auto-ding. Nobody here can give you a definitive idea of your chances to get in, because it is so completely random and case-by-case, but I would say certainly less than 50%. Chances of receiving an offer are contingent upon A) your relative rank in respect to others who are waitlisted, and B) desirability of the bank & group in question.

Offers have already been extended to top picks, and the bank is waiting for their responses. Chances are low that you get off of the list if it's for GS/MS, because most candidates who get offers at these banks will accept. However if it's for a "lower tier" BB or in a not-so-hot group, odds are better. Best thing you can do is keep interviewing, update your status with HR every few weeks, and leverage any future offers by informing HR about them, if this bank is one of your top choices.

If you're really desperate and want to try a "non-traditional" but somewhat risky approach, you can send some emails to senior people you spoke with during your superday. You would speak to the fact that you have been told you are waitlisted, this bank is your top choice, and you really value the personal connection you made with Bob Barker MD about his dog/ cat / golf handicap. Don't make it too over-the-top or desperate sounding, but it will resonate with those MDs who actually liked you and may be the difference maker for someone phoning HR and pushing you through.