Are you talking about the blue @ sign, whose notification is "Push mail: <account here>"?
This shows when you have one or more accounts configured for push mail. By itself, the icon has no value -- the push mail feature is the real "content" here.
Things to check:
- App settings -> mail, receiving -> IMAP push (scroll down to see it)
- Long press the account -> options and folders -> account options -> IMAP push
- Options and folders -> folders -> make sure you see "IMAP push" under the Inbox. If not, tap the settings (wrench / gear) icon on the right and enable it.