"A Trunk or Treat is a Halloween event that is often church- or community-sponsored. People gather and park their cars in a large parking lot. They open their trunks, or the backs of their vehicles, and decorate them. Then they pass out candy from their trunks. The event provides a safe family environment for trick or treaters."

It is apparently a big thing! I had never heard of it either - probably popular in places that have bigger church parking lots than we do!