I found the following answer posted by @Shog9:
In other words, you're being given a birds-eye view of the actual state of review. It's one thing for the majority of reviewers to plow through their own tasks and move on, but someone needs to keep an eye on things to make sure stuff is actually getting done.
If that number is high or growing, then there's a problem. I've heard this lament from many different sites over the past year: "I do all I can, but there aren't enough of us and it just doesn't seem to have any effect!"
If this is happening on your site, if that number is getting bigger in spite of everything you do... Then it's time to raise a call for action. Don't wait for the cruft to clog the streets before getting folks organized to clean it up.
On the other hand, if it's staying steady at a reasonably-small number (and chances are, you know what "reasonably small" means for your site better than I do), then there's nothing to worry about. In fact, you can probably skip clicking on it entirely unless or until it changes, and get along just fine.
For info, I posted the following comment:
I find this indicator quite confusing. All other notifications follow the same graphical pattern: number in red square for inbox, number in green square for rep, number in blue square for mod actions. All these notifications are actionable, so clicking on them allow me to do something to remove them. Not only the review notification does not show me what I can do (which could be useful), but show me stuff I can't do anything about. This is not consistent, and should be moved away from the main page (perhaps in the tool sections)