Unread message icon is to display the number of unread messages in your inbox on the tab header. This will give you a quick glance at how many unread messages in your inbox.
Step 1: Login your Gmail account (please refer here to open one if you do not have an account yet);
Step 2: In the Gmail account, click the "Setting" button from the right-top corner;
Step 3: Click the "Settings" command from the drop-down list;
Step 4: Click the "Advanced" tab in the settings menu and check "Enable" in the "Unread message icon" section;
Step 5: Click the "Save Changes" at the bottom;
Step 6: The number of unread messages will display in the tab header.