What is Priority Inbox?
Priority Inbox splits your inbox into three sections: “Important and unread,” “Starred” and “Everything else”:
As messages come in, Gmail automatically flags some of them as important. Gmail uses a variety of signals to predict which messages are important, including the people you email most (if you email Bob a lot, a message from Bob is probably important) and which messages you open and reply to (these are likely more important than the ones you skip over). And as you use Gmail, it will get better at categorizing messages for you. You can help it get better by clicking the 
or 
buttons at the top of the inbox to correctly mark a conversation as important or not important. (You can even set up filters to always mark certain things important or unimportant, or rearrange and customize the three inbox sections.)
http://gmailblog.blogspot.com/2010/08/email-overload-try-priority-inbox.html