This is one of the best Gmail Labs ever. Basically it filters out messages that aren’t personal. This is really great, especially for people that get a lot of email, I bet. Pretty much everything not sent by a human is bunged neatly away into the respective Bulk, Notifications, and Forums magilabels, ready for your attention only when you choose to give it.
If you need to, you can add a filter to make sure certain notifications or other Smart stuff does land in your inbox. Don’t edit the SmartLabel Filter unless you intend to, though. It affects everything for that Smart category.
Smart Labels, putting the relevance back into (1).