Google Wave Notifications is a new service Google is starting to roll out in Google Wave which basically makes sure you are aware of what's happening at all times. What's so special about that? Well, according to Google, these "Notifiers" are real people.
I'm sure they will be adding more in the future as the number of wave users increases.
In all seriousness though, Google has actually enabled real notifications. You can now opt to receive email when you have new waves waiting for you. It seems a bit strange, but it's a good migration strategy that will probably be needed for several years if Google wants Wave to succeed.