"When you're visiting sites other than Gmail, it's easy to find out how many unread messages are in your inbox by glancing at the title of your Gmail tab or window. However, if you have a ton of tabs open, or if you use Chrome's Pin Tab feature that hides everything except the tab's icon, it can be tricky to figure out without switching tabs," mentions Gmail's blog.
The counter is supposed to show the number of unread messages from the inbox, but it doesn't always work properly.
If you'd like even more features, Lifehacker suggests to use Minimalist Gmail, a Chrome extension that not only adds an unread counter to Gmail's favicons, but it also lets you customize the navigation bar and remove extraneous features like the "invite a friend" box or the cluttered footer.
{ Thanks, Greg. }