The number shown is the total number of posts on the site with outstanding review tasks. When you click on
review, you're shown the posts that you can review. You don't see posts that you've already interacted with: voted on, answered, commented on, or reviewed from a different review queue. I also believe that the indicator won't show up unless there are three or more such posts.
For example, an answer can appear in the "First Posts", "Late Answers" and "Low Quality" queues; if you review it in the "First Posts" queue, it won't be shown to you again, even though other people could see it in the other queues.
At the time you wrote this question, there were two questions in the "Close" review queue that were contributing to the number in the indicator. I've acted on them, bringing the count down to two, so the indicator should no longer be visible.
Now we just need someone to go find the other two posts and review them too...
For more information: