A Gmail user can quickly see the number of unread emails by enabling Unread message icon available as part of Gmail labs. To enable this feature, logon to Gmail then Mail settings from the Options menu
In the setting screen, click Labs link then scroll down to Unread message icon feature. Select Enable radio option available on the right hand side to enable this feature.
Now the browser tab will start displaying the number of unread messages in your Gmail account.
Also see: How to enable preview pane in Gmail