This indicator is shown only to moderators and users who've gained access to the moderator tools. As such, it behaves in keeping with the design of those tools:
If you poke around in /tools, you'll quickly notice that most of what's found there isn't directly actionable - it's informational! What you do with that information is up to you: hopefully, you've been around long enough now that you have some idea of how you want the site to be run, and are able to make good use of the information presented.
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.