<p>I believe that reflects a kind of "urgency" about attending to the queue. If my recollection is right (it may be a bit off), blank means the queues are empty (or perhaps nearly so), a grey dot means there's a number of items in at least one queue (i.e. a queue needs attention) and red indicates there's at least one "long" queue. </p>

<p>Edit:</p>

<p>There's a pretty complete discussion on the network meta here:</p>

<p><a href="https://meta.stackexchange.com/questions/302226/how-does-the-review-needed-indicator-work-exactly">How does the review-needed indicator work exactly?</a></p>
