It's an opportunity for Windows Phones developers and fans to come together and share ideas and experiences with like minded folk. This is the third (or is it the fourth?) meetup in St. Louis and Nokia has helped sponsor each event. This go around, Nokia is providing a Lumia 900 as a door prize.
The event will start at 12PM and run until 4PM and attendees will be responsible for their food and drinks. If you plan to attend, you can sign up here so organizers will know to reserve enough seats. There is also a discussion going on here in the Windows Phone Central Forums about the meetup in case you have any questions, comments or suggestions.