I found that is a new service in Gmail. Gmail can pick the important email for you. According to the video from Google about Priority Inbox of Gmail, the system will detect the importance of email by whether you reply the sender before. That sounds like not very "smart". But you can have fine tune the email by using plus and minus importance buttons. That is good enough. Well, I don't use my gmail account a lot, so I don't know how's good of this function yet. I think I will write another post later to follow up.