It's an accordian bellows that is supposed to compress and form a tight seal over the fuel filler. Like many CARB mandated devices, it doesn't work worth a damn.
If you google, you'll find many links of complaints that they don't fit various vehicles.
They shouldn't be around much longer. The advent of the capless fuel filler makes them redundant, and not very effective.
I'm happy to be live where I can pull up to the gas station and fill up with leaded 110 on the pump spraying it into a funnel. :lol Technically it's illegal, but I've never, ever, ever seen it be enforced.