As mentioned in a comment this is to draw you attention to the fact that there are relatively many pending review tasks.
Thus, if you see it and are willing to help out click on it to visit one of the review queues.
The details are explained in this answer: How does the review-needed indicator work exactly?
This site is in the "large" site category.
Note that the feature itself that the top-bar nags your when there are many items in the queues is not new to the redesigned top-bar. In the old one there appeared and orange number with the rough number of items to be reviewed indicated.