It means you "have" 6 unread messages in your inbox even if that is not true (sometimes).
This "future" is "broken". At the moment it shows past state eg. you had 6 emails in the past stage.
To explain my quotes for "future" and "broken": this unread-email-indicator works right, only if you never close your Gmail tab. This way it gets updated (with some delay) and it will show the right count of unread emails.
To resolve that
6 to show your actual state you will need to open and close your Gmail 2-3 times (with wait intervals between the opening and closing) until this indicator wakes up and gets updated