Google has discovered that their spam filter is multi-purpose. It's great for eliminating spam in your inbox -- it's been effectively fighting spam for many years. Now, Google is taking their spam filter and putting it in reverse -- figuring out which of your non-spam email you actually care most about.
It's still rolling out, so you probably aren't able to see the feature in action, but this YouTube video will give you an idea of what is behind the latest feature.
The Priority Inbox is going to be really useful for me -- I get tons of email, and most of it is from automated notifications about things. These are the types of things that I really don't care much about, but they are important enough to not filter out completely.
This feature, and the "call anyone" feature that Google has put into Gmail the last week have really taken this service to another level -- it's hard to imagine people still use Yahoo Mail and Hotmail for email anymore.