It means you're ahead of the game and already reviewed everything in the queue. The indicator isn't customized to you beyond recognizing which queues you have access to. As such, active reviewers may have this happen from time to time.
So there is, unfortunately, a bit of noise in the signal. Jon discusses this a bit in his announcement about the feature the very last paragraph here.
Finally, all active tasks are counted for each queue when determining if a queue is in “danger”. Thus, there may be times when the indicator will light up but you’ll be unable to do anything about it—for example, if you’ve already done all the reviews you’re allowed to do for the current day in that queue, or if all the items to be reviewed are your own work.