In a nutshell, the organizers pick a theme.
Quilt shops that want to participate design a long narrow block aka a 'row'.
During the open dates, you visit participating shops and get the pattern for free.
Most quilt shops also sell a kit for their design.
You have to go to the shop to get the pattern, they are not supposed to mail it during the specified dates, although later in the year this restriction is lifted.
In addition, you can purchase a fabric license plate unique to that shop.
You can win a prize if you are the first to complete a quilt with a designated number of rows.
Check the link above for all the details.