As Anyon says, this sounds an awful lot like the department has made their choice, and is waiting for approval of their choice through some administrative framework. If this is the case, the reason for notifying you in the manner that they have may be to urge you not to accept some university's "official" offer without considering this offer, or waiting for it to firm up.
Of course, if I'm misinterpreting this, the next most likely scenario is that you're in a second tier of candidates, and they're waiting to see how many in their first tier accept their offer before offering you something firm.
Although universities really appreciate early acceptances, so they can better understand their needs moving forward, your proper course of action is to do nothing until you start approaching deadlines for your decision. If this hasn't been firmed up as it's getting close to the time you really need to decide, and this non-guaranteed offer is something you might consider after knowing whether you're accepted to any the rest of the programs you applied to, then you should email your contact at this program, tell them you're under deadline, and need a firm offer. At that point, the program may need to circumvent their standard protocols, and directly ask the office who can approve your admission to do so before they're done with their entire process.