A red dot is showing under my review queue, what is it?
Jon Ericson's post in Meta Stack Exchange answers it:
With the rollout of the new top bar, you’ve probably noticed the new review indicator:
So the idea is that when you see the indicator light up, it’s a reminder that there are reviews to be done if you haven’t looked recently.
Each queue may be in one of three states:
No dot (empty queue)
Grey dot (at least one but fewer than N review tasks)
Red dot (N or more review tasks)
Update: the logic for lighting up the review queue icon became more complex, and is still subject to change, so refer to the main post for follow ups.
As a side note from me is that most times I get that red dot in my review icon, I click on it but I am already late for the party (no posts; I think it is due to caching).