Flippant: It means Referee 2 has promised a report on the revised version.
I'd normally say that this probably means that Referee 2's review is overdue, they have been contacted by the editorial office, and they've said it's coming. However, given the short time frame, unless you have submitted to a journal with an extraordinarily fast turnaround, this isn't likely the case.
With your added information, and that only one referee has appeared in the status, that likely means they have accepted the review. The updated status may mean that the referee has given the journal a head's up that the report is coming, but that it's delayed in some way that they know in advance (hence the "promised") and the editor selected the wrong status. It may also mean that something went wrong in the initial dispatch of your manuscript and they had to fix something (hence "revised) - I have, for example, had a supplemental file go missing in a packet sent to me.
But all of this is idle speculation - no one here can really know what it means.
My advice? It's been one day since your paper was sent for review. Stop checking on its status.