If programmers or project managers have made changes to a reported issue, those changes are tracked and available for review by subscribing to the issue. Select an Issue from the list and click on the subscribe button in the upper right. It will then turn blue. You will be notified whenever a change or comment is made on that issue.
A number in the (now) Blue Bell indicates you have 3 updates to Issues to which you have subscribed.
Clicking on the bell will show you those updates.