You can pick up a free "passport" at a few locations in Epcot. The festival center is one - last year it was in the old Wonders of Life pavilion. There are other places where they're distributed, but I don't recall exactly where.
The passport has a page for each booth. When you visit the booth they have a stamp and you can get that page in your passport stamped. It was a little confusing for me as some booths had the stamps at the cash registers, and others had them where you pick up your food.
I think there was also space on the pages where you could write notes - maybe what you tried, what you liked, etc.
It's just a fun little thing to do.