[Jack_]

It's an invite sent out to the press and developers inviting them to attend an event at the Flint Centre in Cupertino, California (which incidentally is the same place Steve Jobs unveiled the first Mac 30 years ago). These September events are now an annual thing and are usually when the latest iPhone model is revealed in time for a Autumn/Fall release. There's also rumours of an Apple designed watch (dubbed the iWatch) being revealed also.

The invites they send out are usually cryptic and have messages and clues hidden in them about what's going to be announced. There's speculation of two different size models of the iPhone 6 being produced and some are saying the two sixes upside down are hinting at that.