The flag icon in the message list is a yes/no indicator that denotes whether the mail message has been flagged. This can be used to indicate an action-needed item or to otherwise distinguish the message or conversation from a large number of other items of lesser importance.
To turn the flag on, select the message or conversation and in the More Actions list, select Add flag.
To turn the flag off, select the message or conversation and in the More Actions list, select Remove flag.