That's to tell you that something has been edited in your article (possibly by Edbot) or something was snipped (taken out) from your hub. You can click on it and it will list the changes. From there you can click on it and find out exactly what was changed.
This is done to either keep the hub up to policy or to prepare it for movement to one of the niche sites. Once you've clicked on it, the read exclamation mark will be removed.